235/65R18 vs 255/50R20
Switching from 235/65R18 to 255/50R20 increases the overall diameter by 0.3 mm (0.0%). That is within the usual ±3% guideline.
| 235/65R18 | 255/50R20 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 235 mm | 255 mm | +20.0 |
| Sidewall height | 152.8 mm | 127.5 mm | -25.3 |
| Rim diameter | 457.2 mm | 508.0 mm | +50.8 |
| Overall diameter | 762.7 mm | 763.0 mm | +0.3 |
| Circumference | 2,396.1 mm | 2,397.0 mm | +0.9 |
| Revolutions | 417 per km | 417 per km | +0.0 |
Speedometer effect
With 255/50R20 fitted on a car calibrated for 235/65R18, 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.
235/65R18 dimensions · 255/50R20 dimensions · Open in calculator