What this answers
A tire reads something like 205/55 R16. This page turns two of those into millimetres, shows how much taller or shorter the new one is, and works out the true speed when the speedometer shows a given number.
How a tire size reads
- 205 — the width, in millimetres.
- 55 — the aspect ratio: the sidewall’s height as a percentage of the width.
- R16 — the rim diameter, in inches.
Sidewall = width · aspect / 100
Diameter = rim · 25.4 + 2 · sidewall
Circumference = π · diameter
An inch is exactly 25.4 mm. The circumference is the circumference of a circle.
The speedometer
A speedometer counts how many times the wheel turns. If it is set up for the first size and you fit a larger one, each turn takes the car further, so the car goes faster than the speedometer says. The true speed is the shown speed times the ratio of the two circumferences.
Worked example
205/55 R16 against 225/45 R17:
- the sidewalls are 112.75 and 101.25 mm
- the diameters are 631.9 and 634.3 mm, a difference of 2.4 mm or 0.38%
- with the speedometer at 100, the true speed is 100 · 634.3 / 631.9 = 100.4
What is not included
- Which sizes your car may run. That is in its papers and not on this page.
- Real tires. The sizes are nominal and unloaded. A real tire differs a little between makers and wears.
- A speedometer that is already off. The page takes it to be right for the current size.
Common questions
Can I type mph?
Yes. The answer is in the same unit as the number you type.