Gear Ratio Calculator
Work out axle ratio and the cruising RPM it produces.
How this calculator works
ratio = ring teeth ÷ pinion teeth · RPM = mph × ratio × 336 ÷ tire diameter
The 336 constant folds together the conversions from miles per hour to inches per minute. This assumes a 1:1 top gear — multiply by your overdrive ratio if the transmission is not direct.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- NIST Special Publication 811 2008 — the 336 constant, which folds together miles per hour to inches per minute and the tyre circumference
The edition matters as much as the number. Jurisdictions adopt code revisions on their own schedule, so verify which one applies where you are building.
Worked example
With ring gear teeth 41 , pinion gear teeth 11 , tire diameter 31 in, cruising speed 70 , this calculator returns 3.73 .
Cruising RPM for common axle and tyre combinations
Open any row to load it into the calculator above — the link carries the numbers with it, so it can go straight to a supplier or a crew.
| Job | Size | Axle ratio |
|---|---|---|
| Stock pickup, highway | 41 × 11 × 31 in × 70 | 3.73 |
| Same truck on 33s | 41 × 11 × 33 in × 70 | 3.73 |
| Regeared for 35s | 43 × 10 × 35 in × 70 | 4.30 |
| Highway-friendly 3.42 | 41 × 12 × 31 in × 70 | 3.42 |
| Low-geared crawler | 47 × 9 × 35 in × 60 | 5.22 |
Assumes a 1:1 top gear. Multiply by the overdrive ratio if the transmission is not direct.
Frequently asked questions
What gear ratio should I run with larger tires?
Taller tires effectively raise the gearing, so you need a numerically higher ratio to compensate. Going from 31 to 35 inch tires typically means moving from 3.73 to 4.10 or 4.56 to restore the original feel.
Does a higher gear ratio mean more power?
More torque multiplication at the wheels, not more engine power. A numerically higher ratio improves acceleration and towing at the cost of higher cruising RPM and worse fuel economy.
How do I find my current gear ratio?
Turn the driveshaft and count how many rotations it takes for the wheels to turn once. Three and three quarter turns means roughly a 3.73 axle. The tag on the differential also carries it, if it survives.
What is a good highway gear ratio?
Something that puts the engine between 1,800 and 2,200 RPM at cruise for a modern petrol engine. Much lower and it lugs; much higher and fuel economy suffers noticeably.