255/70R18 vs 275/55R20
Switching from 255/70R18 to 275/55R20 decreases the overall diameter by 3.7 mm (0.5%). That is within the usual ±3% guideline.
| 255/70R18 | 275/55R20 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 255 mm | 275 mm | +20.0 |
| Sidewall height | 178.5 mm | 151.2 mm | -27.3 |
| Rim diameter | 457.2 mm | 508.0 mm | +50.8 |
| Overall diameter | 814.2 mm | 810.5 mm | -3.7 |
| Circumference | 2,557.9 mm | 2,546.3 mm | -11.6 |
| Revolutions | 391 per km | 393 per km | +2.0 |
Speedometer effect
With 275/55R20 fitted on a car calibrated for 255/70R18, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.8 km/h |
| 80 km/h | 79.6 km/h |
| 100 km/h | 99.5 km/h |
| 120 km/h | 119.5 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.
255/70R18 dimensions · 275/55R20 dimensions · Open in calculator