235/75R15 vs 255/55R18
Switching from 235/75R15 to 255/55R18 increases the overall diameter by 4.2 mm (0.6%). That is within the usual ±3% guideline.
| 235/75R15 | 255/55R18 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 235 mm | 255 mm | +20.0 |
| Sidewall height | 176.2 mm | 140.2 mm | -36.0 |
| Rim diameter | 381.0 mm | 457.2 mm | +76.2 |
| Overall diameter | 733.5 mm | 737.7 mm | +4.2 |
| Circumference | 2,304.4 mm | 2,317.6 mm | +13.2 |
| Revolutions | 434 per km | 431 per km | -3.0 |
Speedometer effect
With 255/55R18 fitted on a car calibrated for 235/75R15, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 50.3 km/h |
| 80 km/h | 80.5 km/h |
| 100 km/h | 100.6 km/h |
| 120 km/h | 120.7 km/h |
Larger tires make the car go faster than the speedometer shows. Most speedometers also read slightly high from the factory.
Before you change size
- Check the sizes approved for your vehicle on the door-jamb placard or in the owner's manual.
- Check clearance to suspension, brakes and arches at full lock and full compression.
- Match or exceed the original load index and speed rating.
- Wider tires need an appropriate rim width; a 255 mm tire has a manufacturer-specified rim width range.
235/75R15 dimensions · 255/55R18 dimensions · Open in calculator