Gas mileage is difficult to predict at times. Many years ago, I swapped a 2.94 or a 2.76 for a 2.45??? It made virtually no difference. In fact, the highway RPM's did not change much. The fairly loose factory torque converter seemed to be slipping more. Later I swapped in a transmission with a lock up converter. That dropped the RPM's a little bit. It did not make much difference in the mileage though. I suspect it was because there was not much difference in the gear ratios. (I primarily did the swap because the new axle was a sure grip.)

In some cases, swapping in an overdrive transmission is a better option than changing the gears in the axle.