255/60R18 vs 275/45R20
Switching from 255/60R18 to 275/45R20 decreases the overall diameter by 7.7 mm (1.0%). That is within the usual ±3% guideline.
| 255/60R18 | 275/45R20 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 255 mm | 275 mm | +20.0 |
| Sidewall height | 153.0 mm | 123.8 mm | -29.2 |
| Rim diameter | 457.2 mm | 508.0 mm | +50.8 |
| Overall diameter | 763.2 mm | 755.5 mm | -7.7 |
| Circumference | 2,397.7 mm | 2,373.5 mm | -24.2 |
| Revolutions | 417 per km | 421 per km | +4.0 |
Speedometer effect
With 275/45R20 fitted on a car calibrated for 255/60R18, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.5 km/h |
| 80 km/h | 79.2 km/h |
| 100 km/h | 99.0 km/h |
| 120 km/h | 118.8 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 275 mm tire has a manufacturer-specified rim width range.
255/60R18 dimensions · 275/45R20 dimensions · Open in calculator