by Tim Plaehn
Jupiterimages / Polka Dot / Getty Images
A vehicle transfers power from the engine through the transmission and differential. The gears of these components result in a wheel speed expressed in revolutions per minute (rpm). From the rpm of the wheel speed, the tire size determines the final speed of the vehicle on the road. The calculations can be applied to any vehicle, from a motorcycle to a semi-trailer.
Find the circumference of the wheel in feet from the diameter. The formula for the circumference is the diameter multiplied by pi. Pi is a mathematical constant and is 3.1416 to four places. For example, if the wheel has a diameter of 30 inches, the circumference would be 94.248 inches. Divide by 12 to get 7,854 feet.
Calculate the revolutions of the wheels per mile by dividing 5280 by the circumference of the tire in feet. The example tire will make 672.3 revolutions per mile.
Calculate speed per minute by dividing wheel speed by tire revolutions per mile. For example, if the wheel speed is 300 rpm, the tire in the example is moving at 0.446 miles per minute.
Multiply the speed in miles per minute by 60 to convert the speed to miles per hour (mph). Our example tire with a 30-inch diameter turning at 300 rpm will have a highway speed of 26.8 mph.
- If the calculations need to be performed multiple times, a spreadsheet can be set up with the calculations. Then it is only necessary to change the tire diameter and the speed in rpm to calculate a new speed on the road.