Tyre size calculator
Compare your current size with the one you are considering: how much the overall diameter changes, the difference as a percentage, and how far the speedometer will read out after the swap.
Current size
205/55 R16 · overall 632 mm · sidewall 113 mm
New size
215/50 R17 · overall 647 mm · sidewall 108 mm
A difference of 2.4 % is inside the usual ±3 % tolerance.
Up to about 3 % is generally treated as acceptable for a replacement size. More than that shifts the speedometer reading and can affect ABS and traction control.
| Overall diameter | 632 mm → 647 mm (+15 mm) |
|---|---|
| Sidewall height | 113 mm → 108 mm |
| Rolling circumference | 198.5 cm → 203.2 cm |
| Difference | +2.4 % |
| Speedometer at a true 90 km/h | shows 87.9 km/h |
This calculation is based on dimensions only. Whether a tyre fits also depends on load index, speed rating, rim width and arch clearance — check those against the vehicle data.
How the calculation works
Sidewall height is width × aspect: for 205/55 that is 205 × 55 % = 113 mm. Overall diameter is the rim in inches × 25.4 plus two sidewalls.
Rolling circumference is overall diameter × π. A speedometer counts wheel revolutions, so a larger tyre covers more ground per revolution and the dial starts reading below your true speed.
This is pure geometry and knows nothing about your car's approved sizes. Check the size against the manufacturer's list before buying — it also states the required load index and rim width.
Frequently asked questions
- How much can the tyre diameter differ?
- The general rule is up to 3 % difference in overall diameter. Within that the speedometer error stays small and the electronics behave as designed. The manufacturer's list of approved sizes has the final say.
- Why does the speedometer read wrong?
- A speedometer counts wheel revolutions rather than measuring true speed. A larger tyre covers more ground per revolution, so your real speed is higher than shown; a smaller tyre gives the opposite error.
- Can I fit a wider tyre than standard?
- A wider tyre needs a suitable rim width and enough clearance in the arch. If the overall diameter stays within 3 % and the rim allows it, this is usually possible — but check the manufacturer's list, and ask us if in doubt.
- Does the calculator tell me whether a tyre fits my car?
- No. It compares the geometry of two sizes. Fitment also depends on load index, speed rating, rim width and arch clearance, none of which we know for your car. Use this to compare sizes, then confirm against the vehicle data.