Last week we did some paint work for a customer and drove his ride back to him when finished since it was right on the way home. It has a bone stock 235 with the old electric three speed overdrive transmission. I noticed on a small tach it has that 60mph was only 1500 rpm. In talking to him I learned that it had a 3.08 rear end. I was astonished because this is a 2.16 final gear ratio when in OD. I engaged the OD at about 40mph and it pulled very steadily and smoothly to 60. The route followed was somewhat hilly ( I can hear all the laughing at our Texas "hills" ) but it did not lug or lose speed. The common thought has always been to not go much shallower than a 3.50 rear gear but unless one hauls loads or pulls a trailer the old 235 can handle a much shallower gear than that. There was no clutch release stalling or lurching so in the world of $2.25 or more gasoline this might be something to keep in mind.
See ? I told you so ! . Those trannies are actually mechanical overdive by Borg Warner , they have planetary and sun gears and the electrics are a solenoid to lock the ring gear and a governor to prevent it from engaging at to low a road speed . Disconnect the governor and you can climb hills in 2nd gear over drive really well . -Nate