Larry

How fast do you want to drive?

Having a 59 gmc, with an OD 5 speed, I can offer my thoughts

3.73:1 rear gears with a 0.73 OD and 28 inch tires was a pretty good combination on my 59. I recently changed to 31 inch tires with the 3.73 gears, and I feel like I have lost some of the grunt the truck had. I will likely drop the gears down to 4.11:1 in the near future. I have a 9 inch rear in my truck, about 325 hp from a 447 inch v8 and a 5 speed NV4500.

Currently my truck will do 70 MPH at 2050 rpm. Changing to 4.11 gears will add 220 rpm or so to put me just under 2300rpm at 70mph. The current gearing will not pull the OD on an incline at 60mph.

I would reccomend gears in the 3.73-4.11 range and tires in the 28-31 inch diameter range with a 27 to 30%od (0.73:1 to 0.70:1). I would stay on the lower side of the gear range with a 6cylinder motor.