Revolutions per minute (RPM), the engine speed, measures how quickly a vehicle's gears rotate. This value directly relates to the vehicle's total linear speed, and it relates inversely to the tire's size. The axles of a car with larger tires rotate less speedily, assuming that the vehicle velocity is constant. Derive engine speed from the tire size and the vehicle speed.
Multiply the vehicle's speed in miles per hour by 1,056. For instance, if it travels at 50 miles per hour:
50 * 1,056
= 52,800
Divide the answer by pi.
52,800 / 3.14159
= 16,807
Divide the result by 2:
16,807 / 2
= 8,403
Divide the answer by the tire radius, in inches. If the tire measures 8 inches in radius:
8,403 / 8
= 1,050
This answer is the engine speed in RPM.
