How to Calculate the Speed From Gear Rates

A car's gear rate is the means of stating how its gears interrelate and affect each others' speeds. It describes how the rotational speed of the engine's gears, also known as the engine speed, and the rotational speed of the wheels' axles relate. The axle speed determines the vehicle's speed. Calculate the vehicle's linear speed using the gear rate, the engine speed, and the size of the tires.

Instructions

1

Divide the engine speed, in revolutions per second, by the gear rate. If the engine speed, is 4,500 RPM, for example, and the gear rate is 5.25, then the formula would be: 4,500 / 5.25 = 857.

2

Multiply the result in Step 1 by the tire radius, in inches. If the tires have a 10-inch radius, the formula would be: 857 / 10 = 8,570.

3

Multiply the result of Step 2 by 2. The formula would be: 8,570 x 2 = 17,140.

4

Multiply the result of Step 3 by Pi. The formula would be: 17,140 x 3.14 = 53,846.

5

Divide the answer from Step 4 by 1,056. The formula would be: 53,846 / 1,056 = 51. The resulting answer is the vehicle's speed in miles per hour.