How to Calculate Speed With Tire Diameter & Gear Ratio

Measure the diameter of your tire with a tape measure in inches. Start from any edge of the tire and measure directly through to the opposite side. For the example, use the tire diameter of 18 inches.

Find the gear ratio. Count the teeth of the larger gear, known as the idler gear, and the smaller gear, also called the pinion. For example, if the idler gear has 30 teeth and the pinion has 20, dividing the idler by the pinion gives the value of 30/20 = 1.5. The gear ratio would be 1.5 to 1 or written as 1.5:1. The gear ratio means that for every 1 rotation the idler gear makes, the pinion gear makes 1.5 rotations.

Find your engine's final ratio. The final ratio represents how many times the engine turns in the same time the transmission makes one full turn. This information can be found in your vehicle manual or through a test at your local repair shop. For this example, use .80 to 1 (.80:1) as the final ratio.

Multiply the engine's final ratio by the gear ratio to find the overdrive ratio. For example, if the gear ratio is 1.5, and the final ratio is .80, the overdrive ratio is 1.5 x .8 = 1.2.

Find the rpm value you want to use for your calculation. For instance, if you want to calculate your speed going at 2,000 rpm, your rpm value would be 2,000.

Calculate the speed with the equation: (rpm x tire diameter) / (final drive ratio x 336) = speed in mph. The "336" value is a constant used in the formula. For example, take the values from Steps 1 to 4 and plug them into the equation to find the speed: (2,000 x 18) / (1.5 x 336) = 71.42 mph.