Gear Ratio Speed Calculator

About this calculator

Road speed at a given engine RPM comes down to how many times the wheels turn for each engine revolution, which is set by the gear ratio and final drive ratio together, and how far the car moves for each wheel turn, which is set by the tire’s rolling circumference (π × tire diameter). The full relationship is roughly speed = (engine RPM × tire circumference × 60) / (gear ratio × final drive ratio × unit conversion factor) — a taller gear (numerically lower ratio) or a larger tire both push the same RPM to a higher road speed, while a shorter (numerically higher) gear ratio does the opposite.

Racers, off-roaders and drivetrain tuners use this math when swapping gears, final drive ratios or tire sizes, because changing any one of those three variables shifts the RPM a car sits at for a given speed — critical for keeping an engine in its powerband, for correcting speedometer accuracy after a tire size change, or for balancing acceleration against top speed and fuel economy.

This calculator takes your engine RPM, gear ratio, final drive ratio and tire diameter, and returns your road speed in both km/h and mph.