🏎️ Speedometer Gear Ratio Calculator
Calculate a vehicle’s road speed from engine RPM, gear ratio, final drive ratio, and tire diameter.
How road speed is calculated from RPM
A vehicle’s road speed depends on engine RPM, how much the transmission gear and final drive (axle) ratios reduce that speed, and how far the vehicle travels for each tire revolution (determined by tire diameter).
How this calculator works
The calculator finds the tire’s circumference (pi x diameter), multiplies engine RPM by 60 to get revolutions per hour, divides by the combined gear ratio (gear ratio x final drive ratio) to get wheel revolutions per hour, then multiplies by tire circumference and converts inches to miles to get miles per hour. This is the standard formula used in automotive gearing and speedometer calibration.
This is a theoretical calculation assuming no wheel slip and a fixed tire diameter — actual speedometer readings can differ slightly due to tire wear, tire pressure, and speedometer calibration tolerances.