Gear Ratio Calculator

A larger ratio (driven ÷ driver greater than 1) does multiply torque - a 5:1 reduction delivers five times the input torque at the output shaft, less friction losses. However, output speed drops by the same factor. If you need high torque and high speed simultaneously, you need a more powerful input source, not just a bigger ratio. A gear set redistributes power between speed and torque but cannot create it.

A compound gear train links two or more gear pairs in series, with the output shaft of the first stage driving the input shaft of the next. The overall ratio is the product of every individual stage ratio. A 4:1 first stage followed by a 5:1 second stage produces 4 × 5 = 20:1 overall. Planetary gearsets are a compact form of compound train: the sun, ring, and carrier can be combined in different ways to produce many distinct ratios from very few parts, which is why automatic transmissions achieve six or eight speeds inside a compact package.

They are reciprocals of each other. If the gear ratio (driven ÷ driver) is 4:1, the speed ratio (output speed ÷ input speed) is 1:4 - the output turns at one-quarter the input speed. Engineers sometimes quote speed ratios for overdrives: a speed ratio of 1.5:1 means the output runs 50% faster than the input, which is a gear ratio of 0.67:1. The torque ratio equals the gear ratio (ignoring losses), so a 4:1 gear ratio is also a 4:1 torque ratio and a 0.25:1 speed ratio.

Overdrive is useful whenever the available input speed is too low for the required output speed, or when you want a prime mover to run at its most efficient RPM while the output runs faster. Common examples include the overdrive top gear of a highway transmission (engine at 1800 RPM, output shaft at 2600 RPM), wind turbine step-up gearboxes (rotor at 15 RPM stepping up to 1500 RPM for the generator), and centrifuge drives. An overdrive stage reduces output torque proportionally to the speed increase, so the driven machine must not require more torque than the ratio allows at the reduced torque level.

Undercutting occurs when the involute profile of a gear tooth is carved away near the root during generation, weakening the tooth and reducing effective working depth. For standard 20-degree full-depth spur gears, the minimum tooth count to avoid undercutting is 17 teeth. If you must use a smaller pinion (12–16 teeth), specify a positive profile-shift coefficient (addendum modification) to restore material to the root area. Alternatively, a 25-degree pressure angle reduces the minimum tooth count to about 14. Helical gears tolerate fewer teeth than spur gears because the helix angle distributes the cut geometry. In automotive and aerospace production, profile-shifted gears are the norm rather than the exception.