[Synchro909]

My phaeton has a 3.78 diff and a 26% O/D. I have done about 30,000 miles towing a camper trailer behind it. We travel at 50 mph on mostly flattish country (Australia is that way!). The camper weighs about the same as the car. While acceleration doesn’t make your face go baggy, the car has done it well.
Oh yeh, the engine has a 5.5 head, downdraught carby, counterbalanced crank,inserts and slightly modified cam.
In a nutshell, I think in a car with a lighter body, the 36% O/D would be fine if you have an improved engine. With a standard engine, I’d stick to 26%. Even nothing changed but a high compression head and you’ll be fine.
All that said, don’t expect your fuel consumption to improve. It takes the same amount of fuel to go (say) 50 mpg whether with or without an O/D but things sure run quieter.