Vadim,

I have a 3.70 final drive in my '75 26 footer and I normally drive between 62 and 65 MPH with no problems. Most 3/4 ton pickups of the same era had 4.10 rear end gears and most 1-Ton pickups were equipped with 4.56 gears, with essentially the same size tires as our GMCs.

The main difference between then and now is the overdrive transmissions on the newer cars. Prior to that, trucks, motorhomes, etc ran at higher RPMs than they do now. Of course, we were lucky to get much more than 100,000 miles out of those older vehicles, but that is only one of the reasons the newer ones last longer.