WheelSignal

WheelSignal

Results printed from https://wheelsignal.com/regear-calculator. Nominal estimates; see assumptions below.

Re-gear calculator for bigger tires

Fitting taller tires makes your gearing taller. Work out the axle ratio that restores your original engine speed, and compare the ratios you can actually buy.

Restore your gearing

Tires
Metric, flotation (33x12.50R17) or a diameter in inches.
Gearing
Ring and pinion / final drive, e.g. 3.73
Separate with commas. Check which ratios exist for your axle.
Cruising RPM (optional)
e.g. 0.75 for an overdrive

Unit choices are saved only if you tick the box, in this browser’s local storage. Measurements are never saved or sent anywhere. A shared link carries the inputs after the # in the address, which browsers don’t send to the server.

Results

Enter the old and new tire sizes and your current axle ratio, or press Try an example, then Calculate.

How to use the re-gear calculator

  1. Enter the original tire and the new tire. Use the same basis for both: two size markings, or two measured diameters.
  2. Enter your current axle ratio. It’s often on a tag on the differential, in the vehicle’s build data or in the owner’s manual.
  3. Optionally list the ratios you’re considering, and your top gear ratio and cruising speed to see RPM.
  4. Press Calculate.

Worked example: 31-inch to 35-inch tires on 3.73 gears

The new tire is 12.9% taller. With the 3.73 gears left in, the truck behaves as if it had 3.30 gears on the original tires. To restore the original engine speed you’d need 3.73 × 35 ÷ 31 = 4.21. If the nearest available ratios were 4.10 and 4.56, 4.10 would leave RPM 2.6% lower than original and 4.56 would make it 8.3% higher.

Formulas used

engine revs per distance ∝ axle ratio ÷ tire diameter restoring ratio = current ratio × new diameter ÷ old diameter effective ratio = current ratio × old diameter ÷ new diameter RPM change (%) = (option × old ÷ new − current) ÷ current × 100 cruise RPM = speed ÷ (π × diameter) × top gear × axle ratio

For how gear ratios, tire size and RPM interact, read Gear ratios, tire size and RPM. To see speed in every gear, use the gear ratio calculator.

Limitations

  • Nominal diameters are used for size markings; loaded rolling diameter is usually a little smaller.
  • Assumes no slip in clutches, torque converters or tires.
  • Doesn’t say which ratios exist for a given axle, or recommend a ratio, engine speed or setup.

Frequently asked questions

Why do bigger tyres need different gears?

A taller tyre travels further per revolution, so for the same road speed the wheels, and therefore the engine, turn more slowly. That’s the same as fitting a numerically lower (taller) axle ratio. Re-gearing with a numerically higher ratio brings engine speed back to where it was.

Should I pick the next ratio above or below the restoring ratio?

That’s your call. A ratio above the restoring figure gives slightly higher RPM than original (shorter gearing); one below gives slightly lower RPM. The table shows exactly how far each available ratio lands from your original gearing. Which ratios are made for your axle, and what suits your engine and use, are questions for the axle maker or a drivetrain specialist.

Which tyre sizes can I enter?

Metric sizes (265/70 R17, including LT), flotation sizes (35x12.50R17), or a plain overall diameter in inches such as 33. Flotation sizes use the first number as the nominal diameter.

Does this account for my speedometer?

No. Re-gearing changes engine RPM but not speedometer calibration. On many vehicles the speedometer still needs recalibrating for the new tyre size; see the speedometer error calculator.

Does it work with nominal or measured diameters?

Either, as long as both sizes use the same basis. Real tyres often measure a little smaller than nominal, especially loaded, so measured diameters (or published revolutions per mile) give a better match.

Last updated .