Tyre Size Comparison
Tyre size is read like this: 195/65 R15 means width 195 mm, sidewall height 65% of the width, and a 15 inch rim. Enter the old and new size — you get the diameter, circumference and speedometer difference in percent.
Enter values — the result shows live here.
How to use
- Enter the old tyre width, aspect ratio and rim size (written on the side of the tyre).
- Enter all three parts of the new tyre.
- If the diameter difference is over 3%, it is usually avoided — a warning appears in the result.
Note: Formulas: Sidewall = width x aspect/100 (mm). Diameter = rim x 25.4 + 2 x sidewall (mm). The speedometer is calibrated for the old size; if the new diameter is bigger, your real speed will be higher than the meter shows.
Good to know
- A bigger new diameter makes your real speed higher than the speedometer shows — allow a margin.
- Stay within about 3% diameter change to keep handling, braking and fuel use close to normal.
- Wider tyres can grip better but may rub inside the wheel arch — check clearance first.
- After changing size, get wheel alignment done so the tyres wear evenly.