Tire Size & Speedometer Calculator
Estimate actual speed after a nominal tire diameter change. Adjust the indicated speed and choose mph or km/h in the results.
Where do I find these specs?
Read the tire size on its sidewall. For a wheel marked 7.5J × 17 ET40, enter a width of 7.5 in and offset of 40 mm; 17 is the rim diameter in the tire size. Check wheel markings or product documentation. Include any existing spacer in the current effective offset.
What changes
+2.4 mm taller
Position changes are calculated; fitment is not confirmed. Check actual tire dimensions and clearance through steering and suspension movement. Calculation assumptions ↗
Tire-size comparison
Both setups use the same scale. The schematic shows nominal diameter, rim diameter, sidewall height and section width. Tread and contours are illustrative; actual tire dimensions vary.
Detailed measurements
| Measurement | Existing | New | Change |
|---|---|---|---|
| Overall diameter | 631.9 mm | 634.3 mm | +2.4 mm |
| Section width | 205.0 mm | 225.0 mm | +20.0 mm |
| Sidewall height | 112.8 mm | 101.3 mm | -11.5 mm |
| Circumference | 1985.2 mm | 1992.7 mm | +7.5 mm |
| Revolutions / mile | 811 | 808 | -3 |
How tire size changes estimated speed
Enter the tire size used as the vehicle’s original calibration baseline and the new size, then set an indicated speed in mph or km/h. The estimate is indicated speed × new nominal diameter ÷ original nominal diameter. It assumes the original speedometer read accurately and no recalibration occurred.
Worked example: 205/55R16 to 225/40R18
Nominal diameter increases from 631.9 to 637.2 mm (0.84%). At an indicated 60 mph, estimated actual speed is 60.50 mph. At an indicated 100 km/h, it is 100.84 km/h.
Do larger tires make actual speed higher?
Under this model, yes: a larger tire covers more distance per revolution. A smaller tire produces a lower estimated actual speed at the same indicated reading. Changing rim diameter alone is not enough to determine the effect; compare overall tire diameter.
Does this include the vehicle’s existing speedometer error?
No. Factory calibration, wear, pressure and actual rolling circumference can all affect the reading. If your speedometer already differs from actual speed, this nominal estimate does not account for that difference or calibrate the vehicle.
Will the odometer change too?
If distance is calculated from the same uncorrected wheel-rotation signal, a larger rolling circumference can make recorded distance lower than actual distance. Vehicle systems vary; check the vehicle’s calibration procedure rather than treating this estimate as a correction setting.
Nominal dimension reference: Tire Rack’s tire dimension guide. Compare width and sidewall in the tire size comparison calculator.