Gear Ratio Calculator
Calculate final drive ratio, wheel speed at RPM, and compare gear/diff combos.
How to Use
- Enter tire diameter (in) — work it out from your tire size with the Tire Size calculator.
- Enter axle (final drive) ratio, the transmission gear ratio, and engine RPM.
- Read your speed at that RPM, plus a comparison of common axle ratios at 70 mph.
| Ratio | Best for |
|---|---|
| 2.73 | Tall · max economy |
| 3.08 | Relaxed highway |
| 3.42 | Balanced all-round |
| 3.73 | Sporty / muscle |
| 4.10 | Strong acceleration |
| 4.56 | Towing / off-road |
| 4.88 | Heavy off-road / 35s+ |
Show Work
Formulas
History of the Differential
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.
About This Calculator
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.
About the Gear Ratio Calculator
Working on automotive and vehicle projects? The Gear Ratio Calculator is a free browser tool that gives you the answer in seconds. Calculate final drive ratio, wheel speed at RPM, and compare gear/diff combos.
How it works
Put each value in its box and read the answer as you go. Because it recalculates live, you can play with the inputs to see how each one moves the result — handy for checking your own working or planning ahead. Everything happens on your device, so it is fast and private.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What gear should I drive?
Higher ratio (numerically) = more torque/acceleration, lower top speed. Lower ratio = better highway cruising.
How do I use the Gear Ratio Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Tire upgrade
See how a larger wheel changes your effective gearing.
Diff swap
Compare 3.73 vs 4.10 rear gears.
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