How to Convert Tire Size to RPM Speed

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.

Instructions

1

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

2

Divide the answer by pi.

52,800 / 3.14159

= 16,807

3

Divide the result by 2:

16,807 / 2

= 8,403

4

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.