In every old engine powered OD equipped vehicle I have owned there are times when you have to take it out of OD especially on hills. My son's Stude Wagon originally had 4.27 rear gears and A BW 30% OD. With 28-29" tires 3rd gear direct was 3,000 + rpm at 55mph. We did that from Vegas to Kingman once. In OD the same rpm was around 70. Having a supercharged Stude 289 it rarely ran out of power even loaded and pulling a trailer. My '53 Chevy pickup with a 270 GMC 3.55 rear end, 30" tires, and a T-5 is about 60 mph at 2,700 in 4th direct and 90mph at 3,000 in 5th OD. Rarely have to down shift. (the 90 mph numbers came from averaging several 4-5 mile tests using mile markers and a stop watch on the way home from Bonneville)
The engine will turn 6,000 but maybe not in OD. There is serious air at around 100 mph in those trucks.
