205/55R17 vs 245/40R18
Switching from 205/55R17 to 245/40R18 decreases the overall diameter by 4.1 mm (0.6%). That is within the usual ±3% guideline.
| 205/55R17 | 245/40R18 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 205 mm | 245 mm | +40.0 |
| Sidewall height | 112.8 mm | 98.0 mm | -14.8 |
| Rim diameter | 431.8 mm | 457.2 mm | +25.4 |
| Overall diameter | 657.3 mm | 653.2 mm | -4.1 |
| Circumference | 2,065.0 mm | 2,052.1 mm | -12.9 |
| Revolutions | 484 per km | 487 per km | +3.0 |
Speedometer effect
With 245/40R18 fitted on a car calibrated for 205/55R17, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.7 km/h |
| 80 km/h | 79.5 km/h |
| 100 km/h | 99.4 km/h |
| 120 km/h | 119.3 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.
205/55R17 dimensions · 245/40R18 dimensions · Open in calculator