Smokin 5000HP+ Posting Maniac
Join Date: Feb 2009
TH700R4's And 3.73 Gears
After changing my 3.08 ratio to 3.70's I highly recommend that be done along with every TH700R4 swap.
You see, when the TH700R4's came out in 1982 the induction systems drastically changed to retain good port velocity; the Crossfire manifolds having very small diameter runners and the Tuned Ports also having very small diameter (but 17" long) runners. Because without the change the port velocity would be way too slow at the low rpms the engines ran which would result in very poor fuel mileage.
I have been running my original 3.08's ever since I installed my TH700R4 and my fuel mileage at 50-55 mph was pathetic but very good at 70-75 mph. The fact it improved so much at a higher load and speed indicated my induction system wasn't matched to my transmission's 30% overdrive.
By changing over to 3.70's my cruising rpm increased by 300 rpm and my 50-55 mph fuel mileage increased by 2 mpg.