A vehicle transfers power from the engine through the transmission and differential. The gears in these components result in a wheel speed expressed in revolutions per minute (rpm). From the wheel speed rpm, the size of the tire determines the final road speed of the vehicle. The calculations can be applied to any vehicle, from a bicycle to a semi-truck.
Calculate the circumference of the wheel in feet from the diameter. The formula for circumference is the diameter times pi. Pi is a mathematical constant and is 3.1416 to four places. For example, if the wheel is 30 inches in diameter the circumference would be 94.248 inches. Divide by 12 to get 7.854 feet.
Calculate wheel revolutions per mile by dividing 5,280 by the tire circumference in feet. The example tire will make 672.3 revolutions per mile.
Calculate the speed per minute by dividing the wheel speed by the tire revolutions per mile. For example, if the wheel speed is 300rpm, the example tire is moving at 0.446 miles per minute.
Multiply the miles per minute speed times 60 to convert the speed to miles per hour (mph). Our example tire with a 30 inch diameter turning at 300rpm will have a road speed of 26.8mph.
If the calculations need to be made numerous times, a spreadsheet can be set up with the calculations. Then, only the tire diameter and speed in rpm need to be changed to calculate a new road speed.