245/40R18 vs 245/45R17
Switching from 245/40R18 to 245/45R17 decreases the overall diameter by 0.9 mm (0.1%). That is within the usual ±3% guideline.
| 245/40R18 | 245/45R17 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 245 mm | 245 mm | +0.0 |
| Sidewall height | 98.0 mm | 110.2 mm | +12.2 |
| Rim diameter | 457.2 mm | 431.8 mm | -25.4 |
| Overall diameter | 653.2 mm | 652.3 mm | -0.9 |
| Circumference | 2,052.1 mm | 2,049.3 mm | -2.8 |
| Revolutions | 487 per km | 488 per km | +1.0 |
Speedometer effect
With 245/45R17 fitted on a car calibrated for 245/40R18, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.9 km/h |
| 80 km/h | 79.9 km/h |
| 100 km/h | 99.9 km/h |
| 120 km/h | 119.8 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 245 mm tire has a manufacturer-specified rim width range.
245/40R18 dimensions · 245/45R17 dimensions · Open in calculator