Calculator
Tire Size Calculator
Compare two tyre sizes: overall diameter, sidewall height, revolutions per km and how far your speedometer will be off after the change.
The new tyre is 1.7 % taller: when the speedometer shows 100 km/h, you are really doing 101.7 km/h.
Current 225/45 R17New 235/40 R18Drawn to scale: seen from the side, standing on the same ground · seen from the front
| 225/45 R17 | 235/40 R18 | Difference | |
|---|---|---|---|
| Overall diameter | 634.3 mm | 645.2 mm | 1.72 % |
| Sidewall height | 101.3 mm | 94.0 mm | -7.16 % |
| Section width | 225.0 mm | 235.0 mm | 4.44 % |
| Circumference | 1,992.7 mm | 2,027.0 mm | 1.72 % |
| Revolutions per km | 501.8 | 493.4 | -1.69 % |
Speedometer error
Your speedometer is calibrated for the current size. With the new size, your real speed is:
| Speedometer shows | Actual speed |
|---|---|
| 30 km/h | 30.5 km/h |
| 50 km/h | 50.9 km/h |
| 80 km/h | 81.4 km/h |
| 100 km/h | 101.7 km/h |
| 130 km/h | 132.2 km/h |
How to read a tyre size
In 245/40 R18, 245 is the section width in millimetres, 40 is the aspect ratio (sidewall height as a percentage of width, so 98 mm here), R means radial construction and 18 is the rim diameter in inches. Overall diameter = rim × 25.4 + 2 × width × aspect / 100.
How much difference is acceptable?
Keep the overall diameter within about ±3% of the original. Beyond that the speedometer, ABS and stability control calibration drift, gearing changes noticeably and the tyre may rub on arches or suspension parts. A larger diameter makes the speedometer read low (you drive faster than shown); a smaller one makes it read high. Plus-sizing (bigger rim, lower profile) keeps the diameter while sharpening steering response at the cost of ride comfort and rim protection.
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