In reviewing the EPA's fuel economy numbers, it seems like overdrive often added 2 or 3 mpg. Some cars may do even better, while others may not do as well. It is a bit difficult to make comparisons because often the car manufacturers changed the axle ratio when adding overdrive.

I've heard of some folks losing MPG's by swapping an overdrive transmission into an older car. I suspect in these cases the car's carb had been running on the main circuit before the swap. After installing the overdrive, the lower RPM made the carb run on it's idle circuit. If the idle circuit is to rich, fuel mileage may not improve or could even be lowered.

The lock up torque converter becomes more important the slower the engine turns. I had a 1980's car back many years ago that had 2.4? gears in the rear axle. The original 3 speed automatic transmission had a locking converter. When that transmission went bad, I swapped in an older transmission that did NOT have a lock up converter. If I remember correctly, the highway cruising RPM's increased by 300 - 400, and the fuel economy dropped a bit.