215/65R17 vs 235/55R18
Switching from 215/65R17 to 235/55R18 increases the overall diameter by 4.4 mm (0.6%). That is within the usual ±3% guideline.
| 215/65R17 | 235/55R18 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 215 mm | 235 mm | +20.0 |
| Sidewall height | 139.8 mm | 129.2 mm | -10.6 |
| Rim diameter | 431.8 mm | 457.2 mm | +25.4 |
| Overall diameter | 711.3 mm | 715.7 mm | +4.4 |
| Circumference | 2,234.6 mm | 2,248.4 mm | +13.8 |
| Revolutions | 448 per km | 445 per km | -3.0 |
Speedometer effect
With 235/55R18 fitted on a car calibrated for 215/65R17, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 50.3 km/h |
| 80 km/h | 80.5 km/h |
| 100 km/h | 100.6 km/h |
| 120 km/h | 120.7 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 235 mm tire has a manufacturer-specified rim width range.
215/65R17 dimensions · 235/55R18 dimensions · Open in calculator