Calculate tyre diameter, circumference, revolutions per mile and theoretical road speed from tyre size.
Worked example
For 235/40 R18, the sidewall is 94 mm and the nominal overall diameter is about 645.2 mm before allowing for tread wear, load or manufacturing variation.
Nominal size versus rolling circumference
The calculated circumference is geometric. A loaded tyre deforms and the effective rolling radius can differ from the unloaded dimensions, so real speedometer calibration and gearing calculations may not match perfectly.
Use this before a tyre-size comparison
If you are changing size, use the dedicated comparison calculator to show percentage diameter and speedometer differences directly.
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