235/65R18 vs 275/45R20
Switching from 235/65R18 to 275/45R20 decreases the overall diameter by 7.2 mm (0.9%). That is within the usual ±3% guideline.
| 235/65R18 | 275/45R20 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 235 mm | 275 mm | +40.0 |
| Sidewall height | 152.8 mm | 123.8 mm | -29.0 |
| Rim diameter | 457.2 mm | 508.0 mm | +50.8 |
| Overall diameter | 762.7 mm | 755.5 mm | -7.2 |
| Circumference | 2,396.1 mm | 2,373.5 mm | -22.6 |
| Revolutions | 417 per km | 421 per km | +4.0 |
Speedometer effect
With 275/45R20 fitted on a car calibrated for 235/65R18, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.5 km/h |
| 80 km/h | 79.2 km/h |
| 100 km/h | 99.1 km/h |
| 120 km/h | 118.9 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 275 mm tire has a manufacturer-specified rim width range.
235/65R18 dimensions · 275/45R20 dimensions · Open in calculator