235/40R18 vs 235/45R17
Switching from 235/40R18 to 235/45R17 decreases the overall diameter by 1.9 mm (0.3%). That is within the usual ±3% guideline.
| 235/40R18 | 235/45R17 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 235 mm | 235 mm | +0.0 |
| Sidewall height | 94.0 mm | 105.8 mm | +11.8 |
| Rim diameter | 457.2 mm | 431.8 mm | -25.4 |
| Overall diameter | 645.2 mm | 643.3 mm | -1.9 |
| Circumference | 2,027.0 mm | 2,021.0 mm | -6.0 |
| Revolutions | 493 per km | 495 per km | +2.0 |
Speedometer effect
With 235/45R17 fitted on a car calibrated for 235/40R18, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.9 km/h |
| 80 km/h | 79.8 km/h |
| 100 km/h | 99.7 km/h |
| 120 km/h | 119.6 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 235 mm tire has a manufacturer-specified rim width range.
235/40R18 dimensions · 235/45R17 dimensions · Open in calculator