Tyre size calculator
Compare two tyre sizes: will the speedometer still be right, and how much bigger is the wheel?
Example: 205/55 R16 to 225/45 R17: the overall diameter changes by 0.38%, so at a real 100 km/h the speedometer shows about 99.6 km/h.
Choose the current size (marked *) to see its dimensions. Add a new size to compare.
Overall diameter difference (= speedometer error)
-%
The new tyre is larger. The speedometer shows less than your real speed: when it shows 100 km/h you drive about - km/h.
The new tyre is smaller. The speedometer shows more than your real speed: at a real 100 km/h it shows about - km/h.
The overall diameter is practically the same, so the speedometer is not affected.
same size in practice within tolerance outside tolerance Within the usual +-2% rule of thumb for replacement sizes. Outside the usual +-2% rule of thumb. Check the sizes approved for your car before buying.
Overall diameter
- mm
Add a new size to see the speedometer difference.
Dimensions and notes
| - | - | |
|---|---|---|
| Width (mm) | - | - |
| Profile (%) | - | - |
| Rim (") | - | - |
| Sidewall (mm) | - | - |
| Overall diameter (mm) | - | - |
| Circumference (mm) | - | - |
| Revs per km | - | - |
Differences: width - mm, sidewall - mm, rim - inch, overall diameter - mm.
- The +-2% limit is a rule of thumb from tyre guides, not a law: keep the overall diameter within about that much of the original size.
- The legal rule (UN ECE Regulation 39) concerns the speedometer: it must never show less than the real speed and at most 10% + 4 km/h more.
- Pure geometry from the sizes you enter. The size must also be allowed for your car; check the load and speed index and the manufacturer data.