Gear Ratio Calculator for Bigger Tires (Off-Road)
Enter your axle ratio and tire sizes to see the effect of a tire change.
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Gear Ratio Calculator for Bigger Tires (Off-Road) by TheCalcTools
How the Gear Ratio Calculator for Bigger Tires (Off-Road) works
Enter the axle ratio, the transmission’s top-gear ratio, and the stock and new tire diameters.
A bigger tire acts like a taller gear: the effective ratio is the axle ratio × stock diameter ÷ new diameter.
To restore stock gearing, multiply the axle ratio by new ÷ stock diameter. RPM = mph × axle × top gear × 336 ÷ tire diameter.
The formula
Worked example
Using the default values shown in the calculator:
| Axle (final drive) ratio | 3.73 |
|---|---|
| Transmission top gear ratio | 0.75 |
| Stock tire diameter (in) | 29.5 |
| New tire diameter (in) | 33 |
| Speed on the speedometer (mph) | 65 |
- 29.5 in to 33 in tires change the effective 3.73 axle to 3.33. You would need 4.17 gears to restore stock.
- At a speedometer reading of 65 mph the engine turns 2,071 rpm on stock tires and 1,851 on the new ones, and the true speed is 72.7 mph.
Result:
| Effective axle ratio with new tires | 3.33 |
|---|---|
| Axle ratio needed to restore stock | 4.17 |
| Engine RPM, stock tires | 2,071 |
| Engine RPM, new tires | 1,851 |
| True speed when the speedometer reads the set speed (mph) | 72.7 |
| Speedometer error | 11.9% |
Frequently asked questions
Do I need to regear for 33 inch tires?
Many off-roaders regear when going up 10% or more in diameter, to restore acceleration and towing power and keep the transmission out of too-low RPM.
Will my speedometer be wrong?
Yes. Bigger tires travel farther per revolution, so the true speed is higher than the speedometer reads, unless you recalibrate.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.