Calculators · Tyres and gearing
Tyre size calculator and comparison
A tyre size such as 225/45R17 encodes the tyre’s width, sidewall height and rim diameter. From it you can work out the overall diameter, circumference and how many times the tyre turns per kilometre. Changing tyre size changes all of these, which shifts ride height, gearing and what the speedometer shows. These calculators decode a size, compare two sizes and show the resulting speedometer error.
Tyre dimensions from the size code
Decodes a tyre size into sidewall height, overall diameter, circumference and revolutions per kilometre and mile. These are unloaded figures; a tyre under load rolls on a radius typically 2–3% smaller.
sidewall = width × aspect / 100 diameter = rim × 25.4 + 2 × sidewall circumference = π × diameter
Compare two tyre sizes
Shows how a new tyre size differs from the current one in diameter, circumference and ride height. Most manufacturers recommend keeping the overall diameter within about 3% of the original size.
change % = (D_new − D_current) / D_current × 100 ride height change = (D_new − D_current) / 2
Speedometer error after a tyre change
The speedometer counts wheel revolutions, so a larger tyre makes the car travel further than the speedometer thinks. This calculator shows your true speed for any indicated speed after changing size.
actual = indicated × D_new / D_current
Worked example: 225/45R17 to 225/50R17
- Current sidewall: 225 × 0.45 = 101.25 mm; diameter 17 × 25.4 + 2 × 101.25 = 634.3 mm.
- New sidewall: 225 × 0.50 = 112.5 mm; diameter 431.8 + 225 = 656.8 mm.
- Difference: +22.5 mm, or +3.55% — just over the usual 3% guideline.
- At an indicated 100 km/h the car is really doing 100 × 656.8 / 634.3 = 103.5 km/h.
Overall diameter of common tyre sizes (unloaded)
| Tyre size | Sidewall | Overall diameter |
|---|---|---|
| 205/55R16 | 112.8 mm | 631.9 mm |
| 225/45R17 | 101.3 mm | 634.3 mm |
| 225/40R18 | 90.0 mm | 637.2 mm |
| 235/35R19 | 82.3 mm | 647.1 mm |
| 265/70R17 | 185.5 mm | 802.8 mm |
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About these results
Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.
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