Diff Ratio Calculator
The differential (final drive) ratio is simply the ring gear tooth count divided by the pinion tooth count: diff ratio = ring ÷ pinion.
Enter Values
How to use this calculator
- Enter the ring (crown wheel) tooth count and the pinion tooth count — read them off the gear or the diff tag.
- Read the diff ratio (e.g. 3.73:1) straight away.
- Optional: add your tyre overall diameter (inches), a road speed (km/h) and your top-gear transmission ratio to see the engine RPM at that speed.
How it works
The differential (final drive) ratio is simply the ring gear tooth count divided by the pinion tooth count: diff ratio = ring ÷ pinion. So a 41/11 crown-wheel-and-pinion set is 41 ÷ 11 = 3.73:1, meaning the pinion (driveshaft) turns 3.73 times for each turn of the wheels. A numerically higher ratio (e.g. 4.10) gives more torque multiplication and quicker acceleration; a lower ratio (e.g. 3.23) gives lower cruise RPM and better fuel economy.
The optional cruise-RPM figure works back from tyre size. Tyre circumference (mm) = diameter (in) × 25.4 × π, wheel revs per kilometre = 1,000,000 ÷ circumference, and engine RPM = wheel revs/min × diff ratio × top-gear transmission ratio. The transmission ratio defaults to 1.00 (a direct top gear); enter your actual overdrive ratio (often 0.7–0.85) for an accurate highway figure. The result assumes no tyre slip and a fixed rolling radius.
Worked example
3.73 diff on 30-inch tyres at 100 km/h. A 41-tooth ring gear on an 11-tooth pinion gives a diff ratio of 41 ÷ 11 = 3.73:1. Add a 30-inch overall tyre diameter, a road speed of 100 km/h and a direct (1.00) top gear, and the calculator returns an engine speed of 2,597 rpm. Swapping to a taller tyre or a numerically lower diff (e.g. 3.55:1) would drop that cruise RPM.
Common mistakes
- Swapping ring and pinion — the larger tooth count is the ring (crown wheel), the small one is the pinion. Dividing pinion by ring gives a ratio below 1, which is wrong for a road diff.
- Leaving the transmission ratio at 1.00 when your gearbox has an overdrive top gear. A 0.80 overdrive lowers the cruise RPM by 20%, so the default figure will read high.
- Using tyre section width or wheel diameter instead of the tyre's overall (rolling) diameter for the cruise-RPM calculation.
Frequently asked questions
How do I find my diff ratio without counting teeth?
Many diffs have a metal tag on the housing or a stamped code that lists the ratio directly, or you can jack up both rear wheels, mark the driveshaft and a tyre, then count driveshaft turns per one wheel turn. If you know the tooth counts, this calculator is exact: ring ÷ pinion.
Is a higher diff ratio better?
It depends on the goal. A numerically higher ratio (4.10 vs 3.73) multiplies torque more, helping acceleration, towing and larger tyres, but raises engine RPM and fuel use at a set speed. A lower ratio does the opposite. Fitting bigger tyres effectively lowers your ratio, which is why many 4WDs re-gear after a tyre upgrade.
Why does the cruise RPM assume a 1.00 top gear?
The diff ratio alone doesn't set engine RPM — the gearbox's top-gear ratio matters too. A direct 1:1 top gear is the neutral assumption; if your car has an overdrive top gear (commonly 0.70–0.85), enter it in the transmission-ratio field for a real highway RPM.
Does this account for tyre slip or driveline loss?
No. It's a geometric calculation from tooth counts and rolling circumference, so real-world RPM can differ slightly due to tyre slip, wear and rolling-radius changes under load. Treat it as a close estimate for gearing decisions.
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Cite this tool
Zerdly. (2026). Diff Ratio Calculator. Retrieved from https://www.zerdly.com/tools/diff-ratio-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.