Calculateur de Comparaison de Tailles de Pneus

Reading a tire size: in 225/45R18, 225 is nominal section width in millimetres, 45 is sidewall height as a percentage of width, R means radial construction, and 18 is wheel diameter in inches.

Each tyre's overall diameter = wheel diameter + 2 × (section width × aspect ratio). The speed error assumes tyre B is fitted but the speedometer remains calibrated for tyre A - actual speed = indicated speed × (diameter B ÷ diameter A).

Worked example: compare the entered original and proposed sizes, then use the diameter-change percentage to judge gearing and clearance while the speed result shows what the vehicle is actually doing at the displayed reference speed.

A diameter change within about 3% is a common screening reference, not a universal fitment rule. Load rating, wheel width and offset, brake clearance, suspension clearance and the vehicle manufacturer's approved sizes still need to be checked.

Tyre dimensions are nominal. Measure the fitted tyre when clearance is close because rim width, pressure, load and manufacturer construction change the real section width and diameter.