Gear Ratio Calculator — Ratio, Speed & Torque

Gear Ratio = Number of teeth on driven gear ÷ Number of teeth on driving gear. If a 12-tooth driving gear meshes with a 48-tooth driven gear, the ratio is 48/12 = 4:1 — meaning the driving gear must rotate 4 times for the driven gear to complete 1 rotation.

Gear ratio and speed are inversely related; gear ratio and torque are directly related (ignoring friction losses). A 4:1 reduction gear ratio means output speed is 1/4 of input speed, but output torque is 4× the input torque (multiplied by the ratio) — this is the basic principle behind gearboxes trading speed for torque, or vice versa.

A reduction gear (ratio > 1:1, e.g. 4:1) reduces output speed while increasing torque — used where you need more turning force, like in a car's first gear or a winch. An overdrive gear (ratio < 1:1, e.g. 0.8:1) increases output speed while reducing torque — used for higher speed at lower engine RPM, like a car's highest gear for highway cruising efficiency.