Work out tyre size comparison instantly with clear inputs, formula shown and shareable results.
Overall diameter is the rim diameter plus twice the sidewall height, and sidewall height is width times the aspect ratio. Changing width therefore changes diameter unless the aspect ratio is adjusted — the basis of plus-sizing calculations.
Tyre diameter
diameter = rim (mm) + 2 x width x aspect ratio / 100
Within 2-3% of original. Beyond that the speedometer reads wrong, ABS and stability calibration drift, and gearing changes noticeably.
Fitting a larger rim with a lower aspect ratio to keep the overall diameter constant. It improves steering response at the cost of ride comfort.