Tire size calculator

Compare one tire size against another and read the diameter change, the speedometer error at a speed you choose, and the clearance change. It sells nothing.

Numbers, then nothing

Compare one size against another

Type the size on the sidewall now and the size you are considering. The answer is the geometry and the speedometer error, and the page ends there.

The P-metric size printed on the sidewall. A leading P or LT is fine.

Same form. Width in millimetres, aspect ratio, then wheel diameter in inches.

The speedometer error is reported at this speed as well as as a percentage.

The answer

The fields already hold a worked example. Press Compare the sizes and the answer appears here — computed in this browser, with every assumption it made listed underneath.

The arithmetic behind this page is written out on how this site works, so you can check it by hand.