225/40R18 vs 225/45R17
Switching from 225/40R18 to 225/45R17 decreases the overall diameter by 2.9 mm (0.5%). That is within the usual ±3% guideline.
| 225/40R18 | 225/45R17 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 225 mm | 225 mm | +0.0 |
| Sidewall height | 90.0 mm | 101.2 mm | +11.2 |
| Rim diameter | 457.2 mm | 431.8 mm | -25.4 |
| Overall diameter | 637.2 mm | 634.3 mm | -2.9 |
| Circumference | 2,001.8 mm | 1,992.7 mm | -9.1 |
| Revolutions | 500 per km | 502 per km | +2.0 |
Speedometer effect
With 225/45R17 fitted on a car calibrated for 225/40R18, 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 | 119.5 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 225 mm tire has a manufacturer-specified rim width range.
225/40R18 dimensions · 225/45R17 dimensions · Open in calculator