275/55R20 vs 275/65R18
Switching from 275/55R20 to 275/65R18 increases the overall diameter by 4.2 mm (0.5%). That is within the usual ±3% guideline.
| 275/55R20 | 275/65R18 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 275 mm | 275 mm | +0.0 |
| Sidewall height | 151.2 mm | 178.8 mm | +27.6 |
| Rim diameter | 508.0 mm | 457.2 mm | -50.8 |
| Overall diameter | 810.5 mm | 814.7 mm | +4.2 |
| Circumference | 2,546.3 mm | 2,559.5 mm | +13.2 |
| Revolutions | 393 per km | 391 per km | -2.0 |
Speedometer effect
With 275/65R18 fitted on a car calibrated for 275/55R20, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 50.3 km/h |
| 80 km/h | 80.4 km/h |
| 100 km/h | 100.5 km/h |
| 120 km/h | 120.6 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 275 mm tire has a manufacturer-specified rim width range.
275/55R20 dimensions · 275/65R18 dimensions · Open in calculator