235/65R17 vs 255/45R20
Switching from 235/65R17 to 255/45R20 increases the overall diameter by 0.2 mm (0.0%). That is within the usual ±3% guideline.
| 235/65R17 | 255/45R20 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 235 mm | 255 mm | +20.0 |
| Sidewall height | 152.8 mm | 114.8 mm | -38.0 |
| Rim diameter | 431.8 mm | 508.0 mm | +76.2 |
| Overall diameter | 737.3 mm | 737.5 mm | +0.2 |
| Circumference | 2,316.3 mm | 2,316.9 mm | +0.6 |
| Revolutions | 432 per km | 432 per km | +0.0 |
Speedometer effect
With 255/45R20 fitted on a car calibrated for 235/65R17, 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.
235/65R17 dimensions · 255/45R20 dimensions · Open in calculator