Everyday
Tire Size and Speedometer Calculator
Compare two metric tire sizes and estimate diameter, circumference, ride-height change, and speedometer error. The calculator uses diameter mm = wheel inches × 25.4 + 2 × section width × aspect ratio. It returns more than one result so you can check the main answer against a useful secondary measure. A larger rolling circumference makes actual speed higher than the indicated speed when calibration is unchanged. Tire model, pressure, load, wheel clearance, gearing, and manufacturer limits require physical verification.
Check the displayed units, assumptions, and rounding before relying on the result.
Calculate and compare
Use the number box for precision or the slider for fast scenario testing.
Scenario results
Diameter change
-0.24%
Original 679.3 mm; new 677.7 mm.
Actual speed
99.76 km/h
At an indicated 100 km/h.
Ride-height change
-0.8 mm
Half the diameter difference.
How the calculation works
Use consistent units and retain full precision until the final display step.
Worked example
Reproduce the displayed scenario, then change one assumption at a time.
Assumptions behind the result
- • Inputs use the units shown beside each control.
- • The displayed formula is applied without hidden market or demographic data.
- • Rounding occurs only for display; calculations keep full numeric precision.
- • A larger rolling circumference makes actual speed higher than the indicated speed when calibration is unchanged.
- • Tire model, pressure, load, wheel clearance, gearing, and manufacturer limits require physical verification.
Mistakes that change the answer
- • Mixing percentages with decimals or mixing incompatible units.
- • Relying on a rounded intermediate value instead of the full result.
- • Changing several assumptions at once instead of testing original width separately.