Difference in trans = better milage

ocdart said

MoparPat -
The final drive gear ratio in both the 904 and 727 trannys is 1.00, so no difference in RPM at 70MPH in 'Drive'. However, the 904 has less HP loss due to lighter internal components.
The standard gear ratios for both the 904 and 727 are as follows:
904 trans
1st: 2.70
2nd: 1.54
3rd: 1.00

727 trans
1st: 2.45
2nd: 1.45
3rd: 1.00

The only way to lower your 70MPH rpms is to go to a higher rear gear or install an overdrive trans.

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Early 904's had the same ratios as 727's. They started using 2.71 low gear sets, I believe, in the early '80's (Forgot exactly when.) But you have to be careful because that's when they started going to lock-up converters in both, and some parts, like the converter, will not interchange. So most, if not all, 2.71 low gear 904's will have lock-up converters. The gears can be swapped into the old cases. I just don't know the little things that need to be checked/changed to do so.

But a later, lock-up converter 904 should give a little better mileage while giving a little better off-the-line accelleration also because of the 2.71 gear!