255/45R20 vs 255/55R18
Switching from 255/45R20 to 255/55R18 increases the overall diameter by 0.2 mm (0.0%). That is within the usual ±3% guideline.
| 255/45R20 | 255/55R18 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 255 mm | 255 mm | +0.0 |
| Sidewall height | 114.8 mm | 140.2 mm | +25.4 |
| Rim diameter | 508.0 mm | 457.2 mm | -50.8 |
| Overall diameter | 737.5 mm | 737.7 mm | +0.2 |
| Circumference | 2,316.9 mm | 2,317.6 mm | +0.7 |
| Revolutions | 432 per km | 431 per km | -1.0 |
Speedometer effect
With 255/55R18 fitted on a car calibrated for 255/45R20, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 50.0 km/h |
| 80 km/h | 80.0 km/h |
| 100 km/h | 100.0 km/h |
| 120 km/h | 120.0 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.
255/45R20 dimensions · 255/55R18 dimensions · Open in calculator