235/55R17 vs 255/35R20
Switching from 235/55R17 to 255/35R20 decreases the overall diameter by 3.8 mm (0.6%). That is within the usual ±3% guideline.
| 235/55R17 | 255/35R20 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 235 mm | 255 mm | +20.0 |
| Sidewall height | 129.2 mm | 89.2 mm | -40.0 |
| Rim diameter | 431.8 mm | 508.0 mm | +76.2 |
| Overall diameter | 690.3 mm | 686.5 mm | -3.8 |
| Circumference | 2,168.6 mm | 2,156.7 mm | -11.9 |
| Revolutions | 461 per km | 464 per km | +3.0 |
Speedometer effect
With 255/35R20 fitted on a car calibrated for 235/55R17, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.7 km/h |
| 80 km/h | 79.6 km/h |
| 100 km/h | 99.4 km/h |
| 120 km/h | 119.3 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 255 mm tire has a manufacturer-specified rim width range.
235/55R17 dimensions · 255/35R20 dimensions · Open in calculator