235/60R18 vs 255/55R18
Switching from 235/60R18 to 255/55R18 decreases the overall diameter by 1.5 mm (0.2%). That is within the usual ±3% guideline.
| 235/60R18 | 255/55R18 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 235 mm | 255 mm | +20.0 |
| Sidewall height | 141.0 mm | 140.2 mm | -0.8 |
| Rim diameter | 457.2 mm | 457.2 mm | +0.0 |
| Overall diameter | 739.2 mm | 737.7 mm | -1.5 |
| Circumference | 2,322.3 mm | 2,317.6 mm | -4.7 |
| Revolutions | 431 per km | 431 per km | +0.0 |
Speedometer effect
With 255/55R18 fitted on a car calibrated for 235/60R18, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.9 km/h |
| 80 km/h | 79.8 km/h |
| 100 km/h | 99.8 km/h |
| 120 km/h | 119.8 km/h |
Smaller tires make the speedometer read higher than the actual speed. 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.
235/60R18 dimensions · 255/55R18 dimensions · Open in calculator