235/55R19 vs 255/45R20
Switching from 235/55R19 to 255/45R20 decreases the overall diameter by 3.6 mm (0.5%). That is within the usual ±3% guideline.
| 235/55R19 | 255/45R20 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 235 mm | 255 mm | +20.0 |
| Sidewall height | 129.2 mm | 114.8 mm | -14.4 |
| Rim diameter | 482.6 mm | 508.0 mm | +25.4 |
| Overall diameter | 741.1 mm | 737.5 mm | -3.6 |
| Circumference | 2,328.2 mm | 2,316.9 mm | -11.3 |
| Revolutions | 430 per km | 432 per km | +2.0 |
Speedometer effect
With 255/45R20 fitted on a car calibrated for 235/55R19, 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 | 119.4 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.
235/55R19 dimensions · 255/45R20 dimensions · Open in calculator