Our project car is running 31" tall rear tires with a 3.55:1 rearend ratio, and a non-overdrive (1:1) ratio transmission. Lets do math. A 31 " diameter tire is 31 x 3.1416=97.3896" circumferance. A mile is 5,280 feet. or x 12=63360 inches. So. in order to travel one mile. the tire has to turn 63360/97.3896=650.6 times. At 60 miles per hour, your car is travelling 1 mile per minute. That means that your tire has to rotate 650.6 times in one minute to reach the speed of 60 miles an hour. If you are using a 3.55:1 rearend ratio, then you multiply the ratio times the number of tire rotations in one minute, so 3.55 x 650.6=2309.6---that is the speed at which your rearend input pinion and driveshaft would be turning. If your car does not have an overdrive transmission, then we can assume that the transmission ratio in high gear is 1:1---this is hard and true for manual transmissions, with a very small percentage of slippage for "non lock-up" transmissions like a powerglide. This means that at 60 miles an hour, your engine will be turning 2309.6 RPM. with 31" diameter tires and a 3.55:1 rearend ratio. If you were to use a 4.11:1 rearend ratio, then the engine would be turning at 4.11 x 650.6=2674 RPM.