Automotive
Tire Size Calculator
Compare tire sizes and the speedometer error from changes.
Switching tire sizes changes overall diameter — which alters speedometer reading and odometer accuracy. Compute the error percentage.
True Speed
103.7 km/h actual
- error
- 3.73% error
- odometer Impact
- Odometer reads low
- legal Note
- Exceeds 3% legal tolerance in many regions
Formula and method
Actual Speed = Shown Speed × (New Diameter / Old Diameter)
Tire diameter changes circumferential rotation per axle revolution; speedometer is calibrated to original.
Worked examples
670mm to 695mm
- Original Diameter (mm):
- 670
- New Diameter (mm):
- 695
- Speedometer Speed (km/h):
- 100
True Speed: 103.7 km/h actual
Assumptions and limits
- Constant gear ratio
This tool provides an educational estimate. Rules, rates and personal circumstances can change the answer. Review the assumptions before relying on a result. Report a correction.
Common mistakes to avoid
- Comparing section width instead of overall diameter.
Frequently asked questions
Acceptable change?
Stay within 3% diameter to avoid speedo and ABS issues.