Your vehicle's engine speed describes the speed at which its gears rotate, per minute. The engine speed leads to a proportional rotational speed in the axles, which in turn translates into the vehicle's linear speed. Automobiles almost always include a dashboard indicator that specifies the engine speed, just as the speedometer specifies the car's linear speed. But you can use other data - such as the linear speed and the gear ratio, along with the size of the car's tires -- to calculate the engine speed manually.
Divide 1,056 by the tire's radius, in inches. If the tire has a 10-inch radius, that would come to 1,056 divided by 10 to equal 105.6.
Multiply that number by the vehicle's linear speed, in miles per hour. If it travels at 55 miles per hour, 105.6 multiplied by 55 would equal 5,808.
Divide that number by the mathematical figure known as pi -- 3.14. For example: 5,808 divided by 3.14 would equal 1,848.7.
Divide that figure by two: 1,848.7 divided by two equals 924.4.
Multiply that figure by your vehicle's gear ratio. If your gear ratio is 5.2, you would multiply 924.4. by 5.2, giving you an engine speed of 4,800 revolutions per minute.
