[PetChemMan]

My 1962 Olds Starfire came from the factory with 3.42 rear axle gears. It has 14 inch. tires, and can cruise at 70 miles per hour without excessive rpm. I think that for the street anything up to a 3.90 ratio is suitable. The extra kick from numerically high gears is great, but of course fuel economy and engine noise and wear are the tradeoffs. Most American V-8s can safely cruise at 3500 rpm , so get out your calculator, measure your tire diameter , and find out what rpm you'll have at your maximum sustained cruising speed. You can assume your torque converter has negligible slip at cruising speed. I ran the numbers out, and you can use the following to calculate vehicle miles per hour. (Assuming that your transmission is in high gear, and has no overdrive).
First, divide engine rpm by the axle ratio to obtain wheel rpm. Then, multiply wheel rpm by tire diameter (in inches) , and then multiply that amount by .002972. That will produce vehicle miles per hour. Lets calculate the following example; Engine rpm = 3500; axle ratio = 3.73; tire diameter = 27 inches. First we find wheel rpm = 3500/3.73 = 938.33 rpm. Then multiply 938.33 times 27 inches times .002972 = 75 miles per hour.