OK, so for a few years now I get occasionally annoyed at the speedo cable on my 73. It acts up and sometimes goes bad. To start at the beginning (bear with me, this is long): I bought the car in 1987, and it had 225-70-15 tires on it, then I went shortly afterward to 255-60-15s. Currently on the rear is 255-65-15s. The rear end ratio is 3.36 I believe, and when it was rebuilt in 1989 I have no reason to suspect it was ever changed, or that it was/is anything other than 3.36, based on options, etc. (L-48, 4-speed, no air, manual brakes). Everything worked fine with the speedometer until a few years ago, when it would sometimes become erratic. I have replaced the cable twice, (actually shortly after I bought the car), and more recently the brown driven gear in the trans about 4 times, each time pulling out the old, chewed up one. Lately it has been eating these pretty regularly, and hence the point of this thread. It seems the real problem has been this little 90 degree adapter (see pics) which loosens up and lets that driven gear lose contact inside the trans, and it gets chewed up. This adapter has been on the car as far as I know since before I bought it. But whenever I talk to other Corvette people, no one seems to know what I mean, or has ever seen or had one on their cars. Anyone know if it's factory original, or could it be added on? I can see that it DOES help to make the bend of the cable much less, and therefore would lessen the chance of it breaking. I finally had enough of it causing my problems, and causing the speedo to jump around and record incorrect mileage. So today I just removed the adapter and ran the cable directly into the 4-speed transmission. It all works great now! But......maybe I am used to it being jumpy, but now the speedo seems to be reading fast! I have a little chart, and using some auto math I found in a book on how to calculate speed, mph, axle ratio, instrument correction, etc. it actually appears I might not be off at all, or very, very little. Nevertheless, my FEELING is that it's reading a little off. I used equations that take into account different tires sizes (which my rears are 28.05" diameter, and the original should be 27.05-27.3", all according to Tire Rack), rear ratios, and the like......and at 2000 rpm I am reading 52mph in fourth gear. I didn't think I was going that fast at that rpm before, but maybe it's just because it's been so long since it worked right. Anyone have an opinion on this? Or info on the adapter? I would guess it is a 1:1 ratio 90 degree adapter, and possibly added by a speedometer shop years before I acquired the car, in an effort to cure the broken speedo cable dilemma. Lots of background info, I know, but I wanted to give as much as I could to help out anyone that might be able to help me out, or have any recommendations or theories as to what I have going on here. Thanks in advance, I would appreciate any info! :thumbsup3:

Big T

On that adapter I'd check the in vs. out ratio. Is it really 1 to 1? When you replace your speedo cables, do you grease it? Try hooking up a drill to the adapter & cable, does the needle bounce?
Can you pace your car to check for speedo accuracy? I have seen those adapters on 4X4 chevy trucks but not sure about vettes.

OK, so for a few years now I get occasionally annoyed at the speedo cable on my 73. It acts up and sometimes goes bad. To start at the beginning (bear with me, this is long): I bought the car in 1987, and it had 225-70-15 tires on it, then I went shortly afterward to 255-60-15s. Currently on the rear is 265-60-15s. The rear end ratio is 3.36 I believe, and when it was rebuilt in 1989 I have no reason to suspect it was ever changed, or that it was/is anything other than 3.36, based on options, etc. (L-48, 4-speed, no air, manual brakes). Everything worked fine with the speedometer until a few years ago, when it would sometimes become erratic. I have replaced the cable twice, (actually shortly after I bought the car), and more recently the brown driven gear in the trans about 4 times, each time pulling out the old, chewed up one. Lately it has been eating these pretty regularly, and hence the point of this thread. It seems the real problem has been this little 90 degree adapter (see pics) which loosens up and lets that driven gear lose contact inside the trans, and it gets chewed up. This adapter has been on the car as far as I know since before I bought it. But whenever I talk to other Corvette people, no one seems to know what I mean, or has ever seen or had one on their cars. Anyone know if it's factory original, or could it be added on? I can see that it DOES help to make the bend of the cable much less, and therefore would lessen the chance of it breaking. I finally had enough of it causing my problems, and causing the speedo to jump around and record incorrect mileage. So today I just removed the adapter and ran the cable directly into the 4-speed transmission. It all works great now! But......maybe I am used to it being jumpy, but now the speedo seems to be reading fast! I have a little chart, and using some auto math I found in a book on how to calculate speed, mph, axle ratio, instrument correction, etc. it actually appears I might not be off at all, or very, very little. Nevertheless, my FEELING is that it's reading a little off. I used equations that take into account different tires sizes (which my rears are 28.05" diameter, and the original should be 27.05-27.3", all according to Tire Rack), rear ratios, and the like......and at 2000 rpm I am reading 52mph in fourth gear. I didn't think I was going that fast at that rpm before, but maybe it's just because it's been so long since it worked right. Anyone have an opinion on this? Or info on the adapter? I would guess it is a 1:1 ratio 90 degree adapter, and possibly added by a speedometer shop years before I acquired the car, in an effort to cure the broken speedo cable dilemma. Lots of background info, I know, but I wanted to give as much as I could to help out anyone that might be able to help me out, or have any recommendations or theories as to what I have going on here. Thanks in advance, I would appreciate any info! :thumbsup3:

Big T


If the 90 degree adapter loosens up, it won't cause your driven gear to get chewed up. Because the housing the driven gear is in keeps the driven gear in place. Your drive gear inside the transmission has very likely slipped out of position, and is the reason why your driven gears are getting destroyed. The drive gear is held in place with a cheap little $3 spring clip, and when the spring clip breaks (which they often do any time a speedometer cable siezes in it's housing), the drive gear moves foreword on the output shaft and no longer meshes with the driven gear properly (it happened to me recently). With the driven gear's housing removed from the transmission, see if you can look up in there to see if the drive gear is dead center with the hole in the transmission's tail housing. If it isn't, it SHOULD be!

I would be willing to bet your drive gear spring clip HAS broken and is your whole problem. Changing the $3 spring clip is a lot of work for sure, as the driveshaft has to be removed, the crossmember taken out, and the tailhousing of the transmission has to be removed. If you have ever siezed a speedometer cable, you most likely broke the spring clip at the same time. Because the cheap little spring clip can't take much torque on it. And with the drive gear loose on the output shaft, it WILL cause an erratic speedometer reading (reading slow). The 90 degree adapter isn't a stock item on a Corvette, but go ahead and use it if you want because they are beneficial at lengthening the speedometer cable's life.

So that's my diagnosis. You HAVE a broken drive gear spring clip! Good luck and let us know what you find..........

Thanks for the replies gents, it helps, and here is some more information. I have not had a broken speedo cable in 20 years. I assume that perhaps a previous owner did have that problem, hence the adapter. As far as I know the adapter is 1:1 ratio, don't know why it wouldn't be. So, speedo cable is no longer an issue, I had mentioned it only to give a bit of background. When I did replace it, I did use a little grease but never overgreased the cable. Like most of us, I have had the chattery, jumping tach cable, too, especially in cold weather. I greased that too and haven't had that problem for over 17 years. The speedo cable would become jumpy and erratic right before losing contact altogether, probably from the adapter backing out inside and the cable loosening from the driven gear,(causing it to wobble and get chewed up?) and this is what I am really complaining about, as far as inaccurate mileage and mph. The housing the driven gear resides in is solid, securely in place, and aligned as well, so far as I can see. The driven gear is the only item to be replaced in those last 17 years, because the cable is OK and the housing on/in the transmission seems to be OK. The adapter, however, is internally loose, and when I removed it once, the gears inside didn't mesh, but when I put everything back together I managed to get them together and JB welded the thing on the outside (more than once-actually every time it happens-and you can still see remnants on the end going toward the tranny) so they would stay together (but the housing itself is tight against the trans). A temporary fix, obviously. It is this loosening inside it I still believe to be the cause. Since I bypassed it, the speedo now moves very smoothly, like it should, and like it always has except for when this adapter loosens up. It has been checked hooked up (by the speedometer when driving) and not hooked up (spinning the cable and watching the speedometer move). I have not yet had the chance to have someone pace me to check my readings, and probably won't for a week. I guess really what I am asking, besides what the adapter is and what it's doing there, is if by my tires, rear end ratio, and mph/tach readings info, if my speedo is reading correctly or does it sound like a high reading? Maybe someone can check for me (with their car and/or math) until I can get another car to run beside me as we check gauges. I'd rather try to fix a problem if it exists now than wait until later when I won't have as much time. And I'd hate to think it's a $3 clip, but it would be typical, eh? I am probably making a bigger deal out of this than is needed, but I don't drive the car much, especially in very warm weather (no A/C), but I will be now that it's getting cooler for the next few months. It is probably this infrequent driving that has my memory of mph vs. tach reading confused, and it has taken about 3 weeks for this latest round to come full circle so no wonder now that it' s working again it seems fast. Just wanting someone else's thoughts I guess was what I am after. Sorry for being wordy.....any other thoughts or opinions? Glad to know it seems the adapter is aftermarket, that is a relief. Thanks again!

Big T

Thanks for the replies gents, it helps, and here is some more information. I have not had a broken speedo cable in 20 years. I assume that perhaps a previous owner did have that problem, hence the adapter. As far as I know the adapter is 1:1 ratio, don't know why it wouldn't be. So, speedo cable is no longer an issue, I had mentioned it only to give a bit of background. When I did replace it, I did use a little grease but never overgreased the cable. Like most of us, I have had the chattery, jumping tach cable, too, especially in cold weather. I greased that too and haven't had that problem for over 17 years. The speedo cable would become jumpy and erratic right before losing contact altogether, probably from the adapter backing out inside and the cable loosening from the driven gear,(causing it to wobble and get chewed up?) and this is what I am really complaining about, as far as inaccurate mileage and mph. The housing the driven gear resides in is solid, securely in place, and aligned as well, so far as I can see. The driven gear is the only item to be replaced in those last 17 years, because the cable is OK and the housing on/in the transmission seems to be OK. The adapter, however, is internally loose, and when I removed it once, the gears inside didn't mesh, but when I put everything back together I managed to get them together and JB welded the thing on the outside (more than once-actually every time it happens-and you can still see remnants on the end going toward the tranny) so they would stay together (but the housing itself is tight against the trans). A temporary fix, obviously. It is this loosening inside it I still believe to be the cause. Since I bypassed it, the speedo now moves very smoothly, like it should, and like it always has except for when this adapter loosens up. It has been checked hooked up (by the speedometer when driving) and not hooked up (spinning the cable and watching the speedometer move). I have not yet had the chance to have someone pace me to check my readings, and probably won't for a week. I guess really what I am asking, besides what the adapter is and what it's doing there, is if by my tires, rear end ratio, and mph/tach readings info, if my speedo is reading correctly or does it sound like a high reading? Maybe someone can check for me (with their car and/or math) until I can get another car to run beside me as we check gauges. I'd rather try to fix a problem if it exists now than wait until later when I won't have as much time. And I'd hate to think it's a $3 clip, but it would be typical, eh? I am probably making a bigger deal out of this than is needed, but I don't drive the car much, especially in very warm weather (no A/C), but I will be now that it's getting cooler for the next few months. It is probably this infrequent driving that has my memory of mph vs. tach reading confused, and it has taken about 3 weeks for this latest round to come full circle so no wonder now that it' s working again it seems fast. Just wanting someone else's thoughts I guess was what I am after. Sorry for being wordy.....any other thoughts or opinions? Glad to know it seems the adapter is aftermarket, that is a relief. Thanks again!

Big T


The adapter IS aftermarket. Because Corvettes never came with them. The only way you're going to chew up one driven gear after another is that broken clip I was telling you about. When the clip breaks, the drive gear moves foreword on the shaft then makes "edge contact" with the driven gears teeth. And after a while, it finally chews the teeth off. Drive gears and driven gears SHOULD last 200,000 to 300,000 miles, and as you'rs isn't lasting very long at all, there's something VERY wrong with the contact between the two. Trust me on this. It IS a broken clip!

Give me your axle ratio and your tire's rolling circumference, and I'll tell you what gears you need to make your speedometer dead accurate. To measure the tire's rolling circumference, put a dab of grease on the tread then roll your car on your driveway far enough to leave two grease spots on the concrete. Measure the distance between the grease spots and that distance is your tire's rolling circumference.

So, this weekend I was able to run by my dad's and have him drive in front of me on the street and on the highway. He has a new 2009 car so I figured the speedo on it would be accurate. Using predetermined signals, I kept pace with him at different speeds and noted what my speedometer said for his actual speed. I recorded RPMs too, on both my factory tach and an AutoMeter that is mounted on the steering column. The two tachs are very close in readings, with a linear difference of 100 greater RPMs on the AutoMeter (stock reads 2000, Autometer reads 2100, etc.) This is what I found: in fourth gear my speed shows 42 for an actual speed of 40, at 1600 RPMs (1700 Autometer). My speedo shows 63 at a real 60, at 2400 (2500), and shows 68 at a real 65, with RPMs of 2600 (2700). Going 74 converts to an actual 70, at about 2850 RPMs (2950 AutoMeter). These readings are very accurately measured, done many times, however it can be the case where there is a half-mile per hour difference and I rounded off, or the tach could be 30-50 RPMs off, also rounded off or due to instrument misreading from my angle (I'm 6'4" tall). These readings are very consistent to what the mathematical equations I used said I should be seeing, so I feel pretty confident in what I have provided. As noted earlier, my rear tires have a diameter of 28.05 inches (originals would have been around 27.4), rear end ratio is 3.36 (though with different size rear tires, 255-65-15, it might be effectively more like 3.25), and I have the brown (18 tooth) driven gear. Not sure if there is anything I can do to reasonably get a more accurate speedo, I don't want to pull MAJOR stuff and pay for what is honestly, not much of a discrepancy in speedo readings. And I have lived with such an inaccuracy for years and it never bothered me much, especially since it gets driven maybe 4k miles a year now. It only gave me problems when that 90 degree adapter would internally separate and now that I have chucked that, it won't be a problem anymore. I can always get another new one if so desired, and I just hope nothing else is causing the speedo to be off. Probably not since it has always been off a little (I think even the AIM lists all possible gear/speed ratios, and my combo has the most discrepency), and when I put these tires on it 8 years ago it has been off this same amount as currently. BUT I will keep an eye on it, and see if that little clip could be the culprit that toobroke mentioned. In any case, at least I now have a chart to keep with me that shows what speed I am REALLY going, and even if the speedo cable breaks, I still have a tach reading for each speed increment to keep me within the law! Plus, I know the adapter IS aftermarket, and apparently a 1:1 ratio based on the same readings of speed before the incidents, and now. Thanks for all the advice fellas, and I will definitely keep it in mind, to keep an eye on things, the next few months as I drive around with my speedometer once again reading dead steady (if slightly off still!)

Big T

So, this weekend I was able to run by my dad's and have him drive in front of me on the street and on the highway. He has a new 2009 car so I figured the speedo on it would be accurate. Using predetermined signals, I kept pace with him at different speeds and noted what my speedometer said for his actual speed. I recorded RPMs too, on both my factory tach and an AutoMeter that is mounted on the steering column. The two tachs are very close in readings, with a linear difference of 100 greater RPMs on the AutoMeter (stock reads 2000, Autometer reads 2100, etc.) This is what I found: in fourth gear my speed shows 42 for an actual speed of 40, at 1600 RPMs (1700 Autometer). My speedo shows 63 at a real 60, at 2400 (2500), and shows 68 at a real 65, with RPMs of 2600 (2700). Going 74 converts to an actual 70, at about 2850 RPMs (2950 AutoMeter). These readings are very accurately measured, done many times, however it can be the case where there is a half-mile per hour difference and I rounded off, or the tach could be 30-50 RPMs off, also rounded off or due to instrument misreading from my angle (I'm 6'4" tall). These readings are very consistent to what the mathematical equations I used said I should be seeing, so I feel pretty confident in what I have provided. As noted earlier, my rear tires have a diameter of 28.05 inches (originals would have been around 27.4), rear end ratio is 3.36 (though with different size rear tires, 255-65-15, it might be effectively more like 3.25), and I have the brown (18 tooth) driven gear. Not sure if there is anything I can do to reasonably get a more accurate speedo, I don't want to pull MAJOR stuff and pay for what is honestly, not much of a discrepancy in speedo readings. And I have lived with such an inaccuracy for years and it never bothered me much, especially since it gets driven maybe 4k miles a year now. It only gave me problems when that 90 degree adapter would internally separate and now that I have chucked that, it won't be a problem anymore. I can always get another new one if so desired, and I just hope nothing else is causing the speedo to be off. Probably not since it has always been off a little (I think even the AIM lists all possible gear/speed ratios, and my combo has the most discrepency), and when I put these tires on it 8 years ago it has been off this same amount as currently. BUT I will keep an eye on it, and see if that little clip could be the culprit that toobroke mentioned. In any case, at least I now have a chart to keep with me that shows what speed I am REALLY going, and even if the speedo cable breaks, I still have a tach reading for each speed increment to keep me within the law! Plus, I know the adapter IS aftermarket, and apparently a 1:1 ratio based on the same readings of speed before the incidents, and now. Thanks for all the advice fellas, and I will definitely keep it in mind, to keep an eye on things, the next few months as I drive around with my speedometer once again reading dead steady (if slightly off still!)

Big T


If you're showing 63 mph at an actual 60 mph, your speedometer is reading exactly 5% fast. Try putting a 19 tooth driven gear in it to slow it down a bit. Or go to a speedometer shop and buy a 90 degree adapter that is corrected 5% (100 turns input to 95 turns output).

According to my calculations, you're running a Warner T-10 with a black 8 tooth drive gear. To correct your speedometer EXACTLY, you need a 19.3 tooth driven gear. As you can't buy a 19.3 tooth driven gear, the best you can do is buy a 19 tooth driven gear. Being only .3 teeth off, your speedometer will read about 1.5% fast at 100 mph. But it would be only about 7/8 mph fast at 60 mph. As about as accurate as you could possibly ask for. And it's better to have your speedometer read slightly fast than slow; helps you avoid getting tickets.



I hope this information helps you Big T

If you're showing 63 mph at an actual 60 mph, your speedometer is exactly 5% off (reading fast). Try putting a 19 tooth driven gear in it to slow it down a bit. Or go to a speedometer shop and buy a 90 degree adapter that is corrected 5% (100 turns input to 95 turns output).

What kind of a transmission are you running? A Muncie? A T10? A Super T10? Is your Corvette a 1973?

I was wondering about a different tooth driven gear, and if I remember correctly a 19 tooth IS offered by Ecklers, and is a lighter color (light tan? natural?).....cheaper too. I had figured the deviation to be about 5%, so thanks for confirming that. I used to have to remove my right side exhaust after the header collector to re-install the adapter, and it was always a pain because the clearance was just that little too tight to orient it and fully tighten it down otherwise, and is another of the reasons I took out the adapter. Removing it though, and also replacing the driven gear, I can do without the removal of the exhaust. It's possible a new adapter if its smaller, could be installed without taking off the pipe. I will heavily consider both alternatives, especially if I ever have a problem. Maybe I will change out the driven gear as a winter project if it strikes me. So I thank you once again for giving me that information, it makes me feel a little easier about things. My Vette IS a 73, and the tranny is a 4-speed manual, Muncie as far as I know as it has not been changed out. I don't think it has ever been rebuilt, at least for sure not since I bought the car 22 years ago. It works and shifts quite well, I have only changed the shifter for a Hurst and replaced the clutch about 3 times (it was beginning to be on the way out when I bought it). I have changed the fluid out with Redline manual trans lube, too.
Thanks again for all your help, I appreciate it and have filed it all away for future reference! :thumbsup3: :rockon6rk:

Big T

I noticed after posting my reply, you had edited yours! So yes, it looks like the 19 tooth driven gear is probably the way to go. And now I am not sure of which transmission I have, I may be wrong but I thought 73s had Muncies. Either way, all the additional info is useful and appreciated! Thanks again!

I was wondering about a different tooth driven gear, and if I remember correctly a 19 tooth IS offered by Ecklers, and is a lighter color (light tan? natural?).....cheaper too. I had figured the deviation to be about 5%, so thanks for confirming that. I used to have to remove my right side exhaust after the header collector to re-install the adapter, and it was always a pain because the clearance was just that little too tight to orient it and fully tighten it down otherwise, and is another of the reasons I took out the adapter. Removing it though, and also replacing the driven gear, I can do without the removal of the exhaust. It's possible a new adapter if its smaller, could be installed without taking off the pipe. I will heavily consider both alternatives, especially if I ever have a problem. Maybe I will change out the driven gear as a winter project if it strikes me. So I thank you once again for giving me that information, it makes me feel a little easier about things. My Vette IS a 73, and the tranny is a 4-speed manual, Muncie as far as I know as it has not been changed out. I don't think it has ever been rebuilt, at least for sure not since I bought the car 22 years ago. It works and shifts quite well, I have only changed the shifter for a Hurst and replaced the clutch about 3 times (it was beginning to be on the way out when I bought it). I have changed the fluid out with Redline manual trans lube, too.
Thanks again for all your help, I appreciate it and have filed it all away for future reference! :thumbsup3: :rockon6rk:

Big T

I noticed after posting my reply, you had edited yours! So yes, it looks like the 19 tooth driven gear is probably the way to go. And now I am not sure of which transmission I have, I may be wrong but I thought 73s had Muncies. Either way, all the additional info is useful and appreciated! Thanks again!

It doesn't matter whether or not you have a Muncie or Warner T-10, as both of them used a black 8 tooth drive gear. The Warner Super T-10 used a 7 tooth drive gear, and using that for calculating, I found you needed a 16 tooth driven gear. As you said you had a 18 tooth driven gear already, I knew you didn't have the Super T-10. So I plugged in the 8 tooth drive gear of the Muncies and Warner T-10's and found you needed a 19 tooth driven gear. Anyway Big T, your problem is solved once you put the 19 tooth gear in. Just 7/8 mph fast at 60 mph. Glad to help you..........

I just finished correcting the speedometer in my '68 a few days ago. It was reading about 15 mph fast at 60 mph, and it required going from a 22 tooth driven gear to a 25 tooth driven gear. I too am reading slightly fast at 60 mph now; about 7/8 mph also. But better to read slightly fast than slow. So as I had it fresh on my mind, it was easy for me to calculate your requirements. I had a very different situation than you have. I have a 3.73 rear end and tiny 22.5" diameter tires. So I needed a driven gear with a LOT more teeth. 6 more than you need. And 25 teeth is as high as they go in those driven gears of that diameter.

How did you measure your tire's diameter? You should measure from the ground up to the center of the axle, then multiply by two. Then multiply that by 3.1416 (to get it's circumference). Then divide that distance into 63,360" (the number of inches in a mile). You'll come out with 750 to 800 revolutions per mile.

As this speedometer error has been so annoying to you, I'd go ahead and fix it as soon as possible. Get it over with. Then when winter arrives, it'll already be done and you can do something else to your car.

As this speedometer error has been so annoying to you, I'd go ahead and fix it as soon as possible. Get it over with. Then when winter arrives, it'll already be done and you can do something else to your car.

Yeah, I probably will go ahead and replace the brown 18-tooth driven gear for the 19-tooth. It will be a fairly simple swap now, without having to remove the exhaust, and since I will be off by just a slight amout as you say, it effectively won't matter. Better than I am now. I SO appreciate your time and effort with the calculations, etc. I got my tire dimensions from Tire Rack although I see now they don't carry that Pirelli tire I have anymore, but I do remember the diameter listed then as being 28.05 inches. I believe that was as mounted on a 7 or 7.5 inch wheel, so mine at 8 would be just a tad off from that. Using your method you listed above, I get just a wee under 28 inches, and allowing a 1/16th inch or so here and there for my measuring error, and taking into consideration the very small amount, if any, of centrifugal force as the tire rotates, I get consistently around 730-750 revolutions per mile with calculating.

Glad you got your speedometer reading corrected, too, althought at 15 mph off at 60, you definitely had a lot more to deal with! At least you are very, very close now, and hopefully soon I can find time to get mine closer as well. I would rather have the 7/8 or so mph difference than the 3 mph I am off now, if for no other reason than more accurate mileage accumulation. Thanks again for all the transmission and gear and calculating info, it has been VERY helpful! :thumbsup3:

Big T

Yeah, I probably will go ahead and replace the brown 18-tooth driven gear for the 19-tooth. It will be a fairly simple swap now, without having to remove the exhaust, and since I will be off by just a slight amout as you say, it effectively won't matter. Better than I am now. I SO appreciate your time and effort with the calculations, etc. I got my tire dimensions from Tire Rack although I see now they don't carry that Pirelli tire I have anymore, but I do remember the diameter listed then as being 28.05 inches. I believe that was as mounted on a 7 or 7.5 inch wheel, so mine at 8 would be just a tad off from that. Using your method you listed above, I get just a wee under 28 inches, and allowing a 1/16th inch or so here and there for my measuring error, and taking into consideration the very small amount, if any, of centrifugal force as the tire rotates, I get consistently around 730-750 revolutions per mile with calculating.

Glad you got your speedometer reading corrected, too, althought at 15 mph off at 60, you definitely had a lot more to deal with! At least you are very, very close now, and hopefully soon I can find time to get mine closer as well. I would rather have the 7/8 or so mph difference than the 3 mph I am off now, if for no other reason than more accurate mileage accumulation. Thanks again for all the transmission and gear and calculating info, it has been VERY helpful! :thumbsup3:

Big T


Big T, the speedometer gear calculations call for tire revolutions per mile. The best way to calculate that is put a dab of grease on the tread of your tire, then roll your car on the driveway far enough to leave 2 grease spots on the driveway. Then measure the distance between the grease spots. THAT is your tire's TRUE rolling circumference (in inches). Then you divide 63,360 (inches in a mile) by that distance. And you'll come out with tire revolutions per mile. Give me that corrected figure and I'll re-calculate the speedometer gears for you again. It only takes me 2 minutes to re-calculate everything.

The tire's mounted diameter isn't an accurate way of measuring what your tire does when it's rolling on the ground. So give me the distance between grease spots and I'll re-figure your driven gear needs again.



The formula goes like this:

Number of drive gear teeth (8) times axle ratio (3.36) times tire revolutions per mile (?). Then divide your answer by 1001. The figure you come up with is the number of driven gear teeth you need. Easy, huh? Just takes 2 minutes to calculate that.

Big T, the speedometer gear calculations call for tire revolutions per mile. The best way to calculate that is put a dab of grease on the tread of your tire, then roll your car on the driveway far enough to leave 2 grease spots on the driveway. Then measure the distance between the grease spots. THAT is your tire's TRUE rolling circumference (in inches). Then you divide 63,360 (inches in a mile) by that distance. And you'll come out with tire revolutions per mile. Give me that corrected figure and I'll re-calculate the speedometer gears for you again. It only takes me 2 minutes to re-calculate everything.

The tire's mounted diameter isn't an accurate way of measuring what your tire does when it's rolling on the ground. So give me the distance between grease spots and I'll re-figure your driven gear needs again.



The formula goes like this:

Number of drive gear teeth (8) times axle ratio (3.36) times tire revolutions per mile (?). Then divide your answer by 1001. The figure you come up with is the number of driven gear teeth you need. Easy, huh? Just takes 2 minutes to calculate that.

OK, so I did the grease spot method, and the best I could get as a true measurement was 85.5 inches as the rolling circumference. It was definitely no less than 85 and no more than 86, I measured more than once and it really looks like it is as exactly close to 85.5 as can be. So, plugging that into the formulas you provided I get 741.05 revolutions per mile, which then equates to 19.9 as the number of driven gear teeth that I need. As a lark I also plugged in 85 inches and 86 inches, to give a little leeway in case my 85.5 inch measurement was off, and revolutions per mile was 745.41 for 85, and 736.74 for 86. Those then equal 20 and 19.7, respectively, as the number of gear teeth needed. So, it looks like any way you slice it, the magic gear tooth number is 20. Still, even 19 would get me closer than I am now, and if my thinking is right, even though 20 is closer, my speedometer would actually read just a tad slow, although even at 60 MPH it couldn't possible be that much off (maybe 1/4 MPH off?). Guess it just boils down to whether I want it to read just a little fast, or just an even smaller amount slow. So, what is your opinion on all this, and do my measurements seem plausible, and my calculations correct? Thanks.

Big T

OK, so I did the grease spot method, and the best I could get as a true measurement was 85.5 inches as the rolling circumference. It was definitely no less than 85 and no more than 86, I measured more than once and it really looks like it is as exactly close to 85.5 as can be. So, plugging that into the formulas you provided I get 741.05 revolutions per mile, which then equates to 19.9 as the number of driven gear teeth that I need. As a lark I also plugged in 85 inches and 86 inches, to give a little leeway in case my 85.5 inch measurement was off, and revolutions per mile was 745.41 for 85, and 736.74 for 86. Those then equal 20 and 19.7, respectively, as the number of gear teeth needed. So, it looks like any way you slice it, the magic gear tooth number is 20. Still, even 19 would get me closer than I am now, and if my thinking is right, even though 20 is closer, my speedometer would actually read just a tad slow, although even at 60 MPH it couldn't possible be that much off (maybe 1/4 MPH off?). Guess it just boils down to whether I want it to read just a little fast, or just an even smaller amount slow. So, what is your opinion on all this, and do my measurements seem plausible, and my calculations correct? Thanks.

Big T


Yes your calculations are 100% correct. Easy to do, huh? Especially when you know the formula and the 100% accurate grease spot trick.

Based on your grease spot measurements, a 20 tooth driven gear appears to be exactly what you need. But it may not be available in the correct .875" outside diameter. You'll have to check with Eckler's to see what's available. By the way, how much are your rear tires worn down? Because for every 1/4" they wear down in outside diameter, it changes your rolling circumference by 3/4" less. And that also changes the formula's outcome slightly. So if you got 85.5" with 1/2 worn tires, it could easily be as much as 86.5" with new tires. If your tires are worn down just a little bit, the 20 tooth gear would be the perfect choice.

Calculating speedometer gears needed is rather fun, huh?

So you need a 20 tooth driven gear to put your speedometer damned near dead on. Is a 20 tooth driven gear made? Does Eckler's happen to stock one? As my speedometer in my '68 was about 15 mph fast at 60 mph, I needed 3 more teeth on my driven gear to correct my speedometer. As soon as I get my car back on the ground (the transmission is out at the moment), I'll see how accurate my speedometer is now. It should be pretty close; reading a bit high (about 1-1/2 mph off at 60 mph).

Yes your calculations are 100% correct. Easy to do, huh? Especially when you know the formula and the 100% accurate grease spot trick.

Based on your grease spot measurements, a 20 tooth driven gear appears to be exactly what you need. But it may not be available in the correct .875" outside diameter. You'll have to check with Eckler's to see what's available. By the way, how much are your rear tires worn down? Because for every 1/4" they wear down in outside diameter, it changes your rolling circumference by 3/4" less. And that also changes the formula's outcome slightly. So if you got 85.5" with 1/2 worn tires, it could easily be as much as 86.5" with new tires. If your tires are worn down just a little bit, the 20 tooth gear would be the perfect choice.

Calculating speedometer gears needed is rather fun, huh?
You know, doing these calculations actually IS fun. Pretty sure I never said math was fun when I was in school! Glad to know I had figured the calculations correctly. Thank you for your help with the formulas. Eckler's DOES carry 18, 19, and 20 tooth gears (and many more) but the 19 and 20 listed don't specifically say they fit 73s with a 3.36 ratio. The 18 that I have does say that. I checked other places online and their descriptions range from "see what color you have and replace with same" to "one extra tooth will change 5 MPH" to "fits 1962-1981 transmissions", so a lot to be determined based on just a description. I will try to investigate further. I checked my records again and my tires are newer and have less miles on them than I thought, so if the formulas spit out say, a 19.7 tooth gear with new tires, then allowing for slight wearing down (like 1000-2000 miles or so), then it will be even closer to a 20 tooth gear the more miles come off and they decrease diameter. The tires would likely dry rot before a 20 tooth gear would then become far enough off to matter!
So you need a 20 tooth driven gear to put your speedometer damned near dead on. Is a 20 tooth driven gear made? Does Eckler's happen to stock one? As my speedometer in my '68 was about 15 mph fast at 60 mph, I needed 3 more teeth on my driven gear to correct my speedometer. As soon as I get my car back on the ground (the transmission is out at the moment), I'll see how accurate my speedometer is now. It should be pretty close; reading a bit high (about 1-1/2 mph off at 60 mph).

Good luck with your project, hope it all works out for you. I am sure you are anxious to get it back on the road. As I said above, Eckler's does stock a 20 tooth gear, so I will look into its size to see if its a direct replacement for my 18 tooth gear. I would think so, but maybe if it doesn't say 73 with 3.36 in the description, it won't. Worst case scenario, if a 19 or 20 tooth driven gear won't drop right in, and I have to stick with the 18, at least I know exactly what is going on and what to expect, and have the knowledge to boot on how to fix or help out on another car someday! You have really helped broaden my knowledge, thanks and it is much appreciated!

Big T

You know, doing these calculations actually IS fun. Pretty sure I never said math was fun when I was in school! Glad to know I had figured the calculations correctly. Thank you for your help with the formulas. Eckler's DOES carry 18, 19, and 20 tooth gears (and many more) but the 19 and 20 listed don't specifically say they fit 73s with a 3.36 ratio. The 18 that I have does say that. I checked other places online and their descriptions range from "see what color you have and replace with same" to "one extra tooth will change 5 MPH" to "fits 1962-1981 transmissions", so a lot to be determined based on just a description. I will try to investigate further. I checked my records again and my tires are newer and have less miles on them than I thought, so if the formulas spit out say, a 19.7 tooth gear with new tires, then allowing for slight wearing down (like 1000-2000 miles or so), then it will be even closer to a 20 tooth gear the more miles come off and they decrease diameter. The tires would likely dry rot before a 20 tooth gear would then become far enough off to matter!


Good luck with your project, hope it all works out for you. I am sure you are anxious to get it back on the road. As I said above, Eckler's does stock a 20 tooth gear, so I will look into its size to see if its a direct replacement for my 18 tooth gear. I would think so, but maybe if it doesn't say 73 with 3.36 in the description, it won't. Worst case scenario, if a 19 or 20 tooth driven gear won't drop right in, and I have to stick with the 18, at least I know exactly what is going on and what to expect, and have the knowledge to boot on how to fix or help out on another car someday! You have really helped broaden my knowledge, thanks and it is much appreciated!



Big T


If the blue 20 tooth driven gear is the same .875" diameter as your present 18 tooth gear, it will fit just fine. Eckler's part number for the 20 tooth gear is #45177. To arrive at the tire revolutions per mile, most people measure from the ground up to the center of the axle, then multiply that by 2 to get the tire's diameter. Then they multiply that by 3.1416 to get the tire's circumference. To me, that's a goofy way to measure it. And is why I started using the grease spot trick many years ago. Sure glad to help you Big T. Maybe some day you can help me..........

If the blue 20 tooth driven gear is the same .875" diameter as your present 18 tooth gear, it will fit just fine. Eckler's part number for the 20 tooth gear is #45177. To arrive at the tire revolutions per mile, most people measure from the ground up to the center of the axle, then multiply that by 2 to get the tire's diameter. Then they multiply that by 3.1416 to get the tire's circumference. To me, that's a goofy way to measure it. And is why I started using the grease spot trick many years ago. Sure glad to help you Big T. Maybe some day you can help me..........

I will absolutely help you out sometime, if it is something I am at all able to share advise/expertise/thoughts with! Anytime, my friend! :thumbsup3:

Big T

I will absolutely help you out sometime, if it is something I am at all able to share advise/expertise/thoughts with! Anytime, my friend! :thumbsup3:

Big T


Just be sure to let us know how you came out after changing over to the new 20 tooth gear. I'm wondering why your speedometer was off in the first place. Do you think someone changed the rear axle ratio at one time? Or maybe changed the transmission? Because the driven gear inside of the transmission should have been the correct gear.

So did you ever get the 20 tooth gear installed? Or did you get busy doing other things?

I haven't got around to putting the 20 tooth gear in, lots of stuff been going on recently(mirror light problem, new dog, pulled muscle at work, etc). I believe the speedometer started being off when I put the slightly taller and different brand tires on it a few years ago, I don't have a reason to believe the rear ratio or transmission has ever been changed. I honestly don't recall if the speedo was off or how much it was before I went with the 255-65-15 tires that replaced the 60 series I had on before. So I assume the tires are the culprit for it being off, and of course the part that bothered me more than that was the plastic gear getting damaged from the adapter. But that part is fixed, so it is just the new driven gear to correct everything. I will post up results when I get that done, may be a couple weeks due to holidays and all.

Big T

I haven't got around to putting the 20 tooth gear in, lots of stuff been going on recently(mirror light problem, new dog, pulled muscle at work, etc). I believe the speedometer started being off when I put the slightly taller and different brand tires on it a few years ago, I don't have a reason to believe the rear ratio or transmission has ever been changed. I honestly don't recall if the speedo was off or how much it was before I went with the 255-65-15 tires that replaced the 60 series I had on before. So I assume the tires are the culprit for it being off, and of course the part that bothered me more than that was the plastic gear getting damaged from the adapter. But that part is fixed, so it is just the new driven gear to correct everything. I will post up results when I get that done, may be a couple weeks due to holidays and all.

Big T


As I am retired and I live all alone up in the mountains, I lose track of holidays. To me a holiday is just another day. I guess we have Thanksgiving coming up soon? My daughter e-mailed me yesterday and told me she'll be bringing me a Thanksgiving turkey dinner and a pumpkin pie. Aren't daughters wonderful?

I haven't got around to putting the 20 tooth gear in, lots of stuff been going on recently(mirror light problem, new dog, pulled muscle at work, etc). I believe the speedometer started being off when I put the slightly taller and different brand tires on it a few years ago, I don't have a reason to believe the rear ratio or transmission has ever been changed. I honestly don't recall if the speedo was off or how much it was before I went with the 255-65-15 tires that replaced the 60 series I had on before. So I assume the tires are the culprit for it being off, and of course the part that bothered me more than that was the plastic gear getting damaged from the adapter. But that part is fixed, so it is just the new driven gear to correct everything. I will post up results when I get that done, may be a couple weeks due to holidays and all.

Big T


Did you get the 20 tooth driven gear yet? Did Eckler's have one in stock?

As I am retired and I live all alone up in the mountains, I lose track of holidays. To me a holiday is just another day. I guess we have Thanksgiving coming up soon? My daughter e-mailed me yesterday and told me she'll be bringing me a Thanksgiving turkey dinner and a pumpkin pie. Aren't daughters wonderful?
Holidays are pretty much another day to me too anymore, I am just glad to be home and in good health. I have a son in college, so kids are great to have!

Did you get the 20 tooth driven gear yet? Did Eckler's have one in stock?
I did get the gear, in fact I got the 20 AND the 19 tooth, just in case the 20 was off, also to have some kind of spare, and the fact it didn't cost any more in shipping. So today was a nice day and I got out there and switched those driven gears. The 18 tooth came out fine, no marks or evidence of any damage, so I was happy that apparently THAT problem is solved. Of course it hadn't been in long either. Anyway, in about 20 minutes I had the new 20 tooth in without a problem (it fits,YAY!) and was driving to check it out. The speedometer didn't exhibit any signs of jumping, also an indication the adapter had been doing the chewing earlier. So, all that was left was just to compare mathematical theory with hard visual evidence. I had taken tach readings at various speeds, as indicated on my speedo and the actual speed of a pace car, recorded them, and checked with formulas to see if everything added up beforehand. It did. So without having another car to pace with me yet, I used those tach readings at different speeds then looked at the speedo to see what my speed showed. In theory it would be dead on, or as close as could be allowing for a sixteenth or so of an inch off in tire measurement, rounding the tach readings off on the chart I carried in the car with me, and the possibility of the tach itself or the speedo being just that hair off in the calibration as they sit in the car, etc. Bottom line: Pretty much dead on, full balls accurate! I did not have a chance to check all speeds (kept it at and under 50) and also a little tough to read tach and speedometer readings while also watching the road. But without a doubt, the speed readings are more accurate according to my charts of tach readings at theoretical actual speeds. If anything, the tach readings may be a bit off due to rounding off and my line of sight to the gauge. Definitely going to have a pace car run with me over the weekend to verify everything, but so far it looks real good! I can't imagine now there could be much if any meaningful variation in actual vs. indicted MPH, and even if so, it WOULD be smaller than what was happening with the 18 tooth gear, and even if it was still pretty off the other way, I have the 19 tooth to split the difference! Planning on hanging onto that one though for an emergency spare, or if I change tire sizes. And hey, even the 18 tooth is still in pristine shape, so it can be used again too! Thanks for bearing with me, and reading along, and thanks a ton to toobroke for the immense help and new knowledge. I will make a final (?) post on all this in a few days after I get another car to run with me.

Big T

Holidays are pretty much another day to me too anymore, I am just glad to be home and in good health. I have a son in college, so kids are great to have!


I did get the gear, in fact I got the 20 AND the 19 tooth, just in case the 20 was off, also to have some kind of spare, and the fact it didn't cost any more in shipping. So today was a nice day and I got out there and switched those driven gears. The 18 tooth came out fine, no marks or evidence of any damage, so I was happy that apparently THAT problem is solved. Of course it hadn't been in long either. Anyway, in about 20 minutes I had the new 20 tooth in without a problem (it fits,YAY!) and was driving to check it out. The speedometer didn't exhibit any signs of jumping, also an indication the adapter had been doing the chewing earlier. So, all that was left was just to compare mathematical theory with hard visual evidence. I had taken tach readings at various speeds, as indicated on my speedo and the actual speed of a pace car, recorded them, and checked with formulas to see if everything added up beforehand. It did. So without having another car to pace with me yet, I used those tach readings at different speeds then looked at the speedo to see what my speed showed. In theory it would be dead on, or as close as could be allowing for a sixteenth or so of an inch off in tire measurement, rounding the tach readings off on the chart I carried in the car with me, and the possibility of the tach itself or the speedo being just that hair off in the calibration as they sit in the car, etc. Bottom line: Pretty much dead on, full balls accurate! I did not have a chance to check all speeds (kept it at and under 50) and also a little tough to read tach and speedometer readings while also watching the road. But without a doubt, the speed readings are more accurate according to my charts of tach readings at theoretical actual speeds. If anything, the tach readings may be a bit off due to rounding off and my line of sight to the gauge. Definitely going to have a pace car run with me over the weekend to verify everything, but so far it looks real good! I can't imagine now there could be much if any meaningful variation in actual vs. indicted MPH, and even if so, it WOULD be smaller than what was happening with the 18 tooth gear, and even if it was still pretty off the other way, I have the 19 tooth to split the difference! Planning on hanging onto that one though for an emergency spare, or if I change tire sizes. And hey, even the 18 tooth is still in pristine shape, so it can be used again too! Thanks for bearing with me, and reading along, and thanks a ton to toobroke for the immense help and new knowledge. I will make a final (?) post on all this in a few days after I get another car to run with me.

Big T


I'm glad it all worked out well for you Big T. Mechanical speedometers are never exactly accurate. They can't be because there's too many variables involved. Just putting an extra 5 psi of air in your tires is enough to change the accuracy of your speedometer. But if you can get a speedometer reading within 1% to 2% of being dead accurate, that's about all you can ask for. If I recall, the 20 tooth driven gear would make your speedometer read about 1/2 mph fast at 60 mph. That means you'd be within about 1% accurate. About as good as it could get. Yeah, let us know how your speedometer does when you compare your car's indicated speed with that of a new car. It should be damned near dead on.

Now that you finally have your speedometer straightened out and your car isn't pulling to the right anymore, what other mechanical issues can I help you with? You can always reach me at sw82vette@yahoo.com if you have any more questions.

I'm glad it all worked out well for you Big T. Mechanical speedometers are never exactly accurate. They can't be because there's too many variables involved. Just putting an extra 5 psi of air in your tires is enough to change the accuracy of your speedometer. But if you can get a speedometer reading within 1% to 2% of being dead accurate, that's about all you can ask for. If I recall, the 20 tooth driven gear would make your speedometer read about 1/2 mph fast at 60 mph. That means you'd be within about 1% accurate. About as good as it could get. Yeah, let us know how your speedometer does when you compare your car's indicated speed with that of a new car. It should be damned near dead on.

Now that you finally have your speedometer straightened out and your car isn't pulling to the right anymore, what other mechanical issues can I help you with? You can always reach me at sw82vette@yahoo.com if you have any more questions.

Re-did some of my calculations, and without rounding off anything, took the car out again on a level road, virtually no traffic, and went at more speed intervals than before. At this point it was still me by myself, no pace car. Having fewer distractions, I was able to use the information to check everything more thoroughly, and I am dead on for correct mph, as far as I can tell! :beerbang: :woowoo: Even allowing for micro amounts of mechanical leeway and such, by my eyes I can not be off more than about 1/2 mph, and as you say, that is about the amount I could expect. So all is well! I had hoped to get another car to pace me, but people are busy this time of year so it may have to wait a few more days. And I am fine with it if that doesn't happen. Regardless, I am overjoyed, and can live with the small (if it even exists) discrepency with the readings on the speedo. Thanks so much Toobroke, you have been a tremendous help, with this and other issues. :thumbsup: Have you gotten your transmission issues fixed yet? Thanks for the offer to help again. Now, if I could just figure out that deal with my mirror light.....LOL, it's probably a pinched wire somewhere along the windshield header where the wiring to the mirror runs!

Big T

Re-did some of my calculations, and without rounding off anything, took the car out again on a level road, virtually no traffic, and went at more speed intervals than before. At this point it was still me by myself, no pace car. Having fewer distractions, I was able to use the information to check everything more thoroughly, and I am dead on for correct mph, as far as I can tell! :beerbang: :woowoo: Even allowing for micro amounts of mechanical leeway and such, by my eyes I can not be off more than about 1/2 mph, and as you say, that is about the amount I could expect. So all is well! I had hoped to get another car to pace me, but people are busy this time of year so it may have to wait a few more days. And I am fine with it if that doesn't happen. Regardless, I am overjoyed, and can live with the small (if it even exists) discrepency with the readings on the speedo. Thanks so much Toobroke, you have been a tremendous help, with this and other issues. :thumbsup: Have you gotten your transmission issues fixed yet? Thanks for the offer to help again. Now, if I could just figure out that deal with my mirror light.....LOL, it's probably a pinched wire somewhere along the windshield header where the wiring to the mirror runs!

Big T


I got my transmission back in but I have the rest of the car torn apart now. I'm in the process of restoring this '68 and I have the interior out of it, both doors off, the headlight assemblies out, front and rear bumpers off, and most of the paint removed. I'm going to re-paint it myself (first time I've ever tried re-painting a car), and after I get it re-painted, I'm going to dive into the rest of the problems. I don't expect to have it running again until spring.

I got my transmission back in but I have the rest of the car torn apart now. I'm in the process of restoring this '68 and I have the interior out of it, both doors off, the headlight assemblies out, front and rear bumpers off, and most of the paint removed. I'm going to re-paint it myself (first time I've ever tried re-painting a car), and after I get it re-painted, I'm going to dive into the rest of the problems. I don't expect to have it running again until spring.

Wow Toobroke, you certainly have a few projects ahead of you to keep you busy! Kudos on doing the work yourself, I hope the paint turns out well! I love the 68's, I had looked at one before I got my 73 but it was a little out of my price range at the time. After 22 years I am sure the price is even more so on those 68's, for sure on ones that are complete and already in great shape. I just don't have the patience anymore probably to tackle a full blown project car, so much respect to you for taking it on and bringing one back!

Ok, so today being Thanksgiving I got a relative with a new car to pace me on the streets and highway, while the traffic was light, and at last I can say I am spot on for accuracy! At MOST, I am about half-mile per hour off at 70! :woowoo: Better than expected and still within the acceptable parameters! Thanks again Toobroke! :yoda: :cheers: :thumbsup: Another project completed! :burnout:

Big T

Wow Toobroke, you certainly have a few projects ahead of you to keep you busy! Kudos on doing the work yourself, I hope the paint turns out well! I love the 68's, I had looked at one before I got my 73 but it was a little out of my price range at the time. After 22 years I am sure the price is even more so on those 68's, for sure on ones that are complete and already in great shape. I just don't have the patience anymore probably to tackle a full blown project car, so much respect to you for taking it on and bringing one back!

Ok, so today being Thanksgiving I got a relative with a new car to pace me on the streets and highway, while the traffic was light, and at last I can say I am spot on for accuracy! At MOST, I am about half-mile per hour off at 70! :woowoo: Better than expected and still within the acceptable parameters! Thanks again Toobroke! :yoda: :cheers: :thumbsup: Another project completed! :burnout:

Big T


So you're now a happy camper? Good for you Big T. You did it! I bought this '68/454"/4-speed for $12,000 and to this date, I have thrown another $5000 in it. And I still need to re-upholster the seats. But when I get done with it, it will be a darned nice car. When I get done restoring it, I have a '71/454"/automatic that's waiting to be restored. These big block Corvettes are FUN to drive because they have so much low end torque. Just awesome pulling power.