[toobroketoretire]

After I installed the TH700R4 in my 454" powered '71 I got 12 mpg at 50 mph but got 16 mpg at 70-75 mph. Logic would tell you the fuel mileage would be better at the lower speed so why did it get better fuel mileage at a higher speed?

It's all about matching components. My engine has a 220/220 duration marine cam so my maximum "lope" rpm is 1300. And at 50 mph my cruising rpm was also 1300. So my engine was running at a very inefficient rpm at the lower 50 mph speed.

By simply changing my rear ratio to 3.70 from the stock 3.08 I increased my 50 mph cruising rpm to 1600; putting my engine into a much better power band and substantially increasing the fuel mileage. And with the 3.70's I can cruise at 70 mph in 4th at 2385 rpm then kick it down into 3rd and be right at where my torque curve starts.

Yes, changing rear end ratios is expensive but absolutely necessary to match the cam's characteristics.

 

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yeah, I went from 2.87 stock ratio , to a 3.31 ,better, but still not right, so, then on to a 3.73, which turned out to be perfect for my combination

 

[toobroketoretire]

Selecting Rear End Gears

yeah, I went from 2.87 stock ratio , to a 3.31 ,better, but still not right, so, then on to a 3.73, which turned out to be perfect for my combination

I fully agree with you Fish. As your beast is a street driven car you had to use caution when selecting a set of gears. Anything lower than 3.73's would have been a bit too low for decent fuel mileage and anything higher would have been a mismatch with your higher torque curve. So selecting rear gears is a "juggling" act that requires some thought and common sense.

The most important thing to consider when selecting gears is what the car will be used for MOST of the time. Yes, the lower 4.11's and 4.56's will give blazing off-the-line performance but will totally destroy any chances of getting acceptable fuel mileage.

With overdrive transmissions you can "have your cake and eat it too" as you can run the lower gears needed for drag racing but still cruise at a comfortable rpm. In my case I can run low 3.70's but still be able to cruise at only 2000 rpm; thanks to my TH700R4.

 

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thats the best part..having your cake and eating it too.. 3.73 s give great low end power for launch and 1/4 mile, and then you just slip it into 4th OD and the TCC locks up....and Voila, I m doing 80 mph and only turning 2550 rpm..with no effort at all from my motor

 

[toobroketoretire]

thats the best part..having your cake and eating it too.. 3.73 s give great low end power for launch and 1/4 mile, and then you just slip it into 4th OD and the TCC locks up....and Voila, I m doing 80 mph and only turning 2550 rpm..with no effort at all from my motor

Well, I have had the chance to put a few miles on my new 3.70's and I'm impressed with the increased performance in every gear. I have to drive a 10 mile long 5-6% grade coming home and now it'll pull that long grade with the greatest of ease in 4th gear. And with a 60 mph cruising speed at 2040 rpm my engine is still loafing along but at a much more efficient rpm.

When the TH700R4's came out in 1982 all of the engines used small-diameter long runners to keep the port velocity high. So with my marine cam and large-runner Edelbrock Air Gap my port velocity was way too slow with my high 3.08 gears. By installing the 3.70's my port velocity increased substantially as did my fuel mileage at all speed ranges.

 

[toobroketoretire]

yeah, I went from 2.87 stock ratio , to a 3.31 ,better, but still not right, so, then on to a 3.73, which turned out to be perfect for my combination

You're absolutely right Fish. The 3.73's is a perfect ratio for TH700R4 usage and I would highly recommend 3.73's be installed any time a TH700R4 is swapped in.