Engineering · 5 min read · Last updated September 2026
Worked example: 20 T pinion at 1,500 rpm → 40 T gear. Ratio = 40 ÷ 20 = 2:1. Output = 1,500 ÷ 2 = 750 rpm. The output shaft turns half as fast with (ideally) twice the torque — power stays constant, which is the whole point.
Multi-stage ratios multiply: a 20:40 stage then a 20:30 stage then a 20:25 stage gives 0.5 × 0.667 × 0.8 = 0.267 — a 3.75:1 overall reduction. Input 1,500 rpm → 400 rpm out. Each mesh adds friction losses (typically a few percent), so real output torque is slightly under the ideal figure.
Ratios below 1 (overdrive, e.g. 0.8:1) raise output speed and cut torque — that is your car's highway gear. Ratios above 1 (reduction) are climbing gears and gearboxes. Bicycle gearing, conveyor speeds and drill chucks all follow the same two-formula logic; the Gear Ratio Calculator handles the arithmetic for any teeth counts.
Divide the driven gear’s tooth count by the driver’s. A 20-tooth driver turning a 40-tooth gear is a 2:1 reduction — output speed halves.
More torque, less speed, when the ratio is above 1 (reduction). Below 1 it is an overdrive: more speed, less torque. Power stays roughly constant either way.
Multiply the stage ratios. Stages of 0.5, 0.667 and 0.8 combine to 0.267 — an overall 3.75:1 reduction.
Friction. Every gear mesh loses a few percent to sliding contact and lubrication drag, so a 2:1 stage multiplies torque by slightly less than 2.