Gear Ratio Calculator

The automotive differential dates to a Chinese south-pointing chariot from around 200 CE, but its modern form was patented by Onésiphore Pecqueur in 1827. Every car since has needed a differential to let the outside wheel turn faster than the inside wheel in turns. Final drive ratios between 3.0:1 and 4.5:1 are typical, balancing acceleration (higher ratio) against highway fuel economy (lower ratio).

The 3.73 and 4.10 gear ratios became iconic because they're the GM/Ford factory options offered in 1960s-70s muscle cars. A 1969 Camaro with the base 3.07 axle hit 60 in 7.8 s; the same car with a 4.10 "performance" axle did it in 6.9 s — at the cost of spinning the engine 33% faster at highway speeds, hurting gas mileage by 3-4 MPG. Modern multi-speed transmissions (8-10 gears) give you both acceleration and cruising efficiency by letting the final drive ratio be deeper and the top gear very tall.

Tire diameter has the same effect as gear ratio. A 33" off-road tire on a truck originally designed for 30" tires effectively reduces the axle ratio by 30/33 = 0.91, slowing the speedometer by 9% and cutting acceleration. Many off-road builds "regear" the axles (install lower-numerically-higher ratios like 4.56 or 4.88) to compensate.

Enter tire diameter (inches), axle/final drive ratio, transmission gear ratio for the gear of interest, and engine RPM. The tool computes final drive (axle × trans), wheel RPM (engine RPM / final drive), and vehicle speed (MPH = π·D·RPM / 336; km/h = MPH × 1.609).

Use this to preview gear changes before buying parts: try several axle ratios (3.08, 3.42, 3.73, 4.10, 4.56) at your typical cruising RPM to see which fits your driving profile. For a manual transmission, evaluate every gear — a deep first gear (3.5:1+) helps launches; a tall overdrive (0.6-0.7:1) saves fuel. Everything runs client-side; no values leave your browser.