255/30R20 vs 245/40R18 tire size comparison
Enter your current size and the size you’re considering. Everything below updates as you change the numbers — diameter, clearance, speedometer error and whether it fits.
Well within tolerance
-1.18% diameterDiameter change is under 1.5%. Speedometer error is negligible and clearance is effectively unchanged.
| Measurement | 255/30R20 | 245/40R18 | Diff | % |
|---|---|---|---|---|
| Overall diameter | 26.02″ | 25.72″ | -0.31″ | -1.2% |
| Section width | 255.0 mm | 245.0 mm | -10.0 mm | -3.9% |
| Sidewall height | 76.5 mm | 98.0 mm | +21.5 mm | +28.1% |
| Rim diameter | 20″ | 18″ | -2″ | -10.0% |
| Circumference | 81.76″ | 80.79″ | -0.96″ | -1.2% |
| Revolutions / mi | 775 | 784 | +9 | +1.2% |
| Ground clearance | — | — | -0.15″ | |
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Speedometer reading
Welcome to our wheel and tire size calculator
Put your current tire size — width, aspect ratio and rim diameter — into the first form, then your desired size into the second. You’ll get the difference between the new and the old size, plus the speedometer reading difference and your actual speed on the new set. Both metric and flotation (LT inch) sizes are supported.
The calculator helps you decide whether a set of tires and wheels will fit your car. To compare tires by brand and model instead, use our tire comparison tool; to look up factory sizes, browse by car make.
Reading a tire size
- 225
- Section width in millimetres
- 45
- Aspect ratio — sidewall height as a % of width
- R
- Radial construction
- 17
- Rim diameter in inches