245/75R17 vs 255/65R18
Switching from 245/75R17 to 255/65R18 decreases the overall diameter by 10.6 mm (1.3%). That is within the usual ±3% guideline.
| 245/75R17 | 255/65R18 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 245 mm | 255 mm | +10.0 |
| Sidewall height | 183.8 mm | 165.8 mm | -18.0 |
| Rim diameter | 431.8 mm | 457.2 mm | +25.4 |
| Overall diameter | 799.3 mm | 788.7 mm | -10.6 |
| Circumference | 2,511.1 mm | 2,477.8 mm | -33.3 |
| Revolutions | 398 per km | 404 per km | +6.0 |
Speedometer effect
With 255/65R18 fitted on a car calibrated for 245/75R17, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.3 km/h |
| 80 km/h | 78.9 km/h |
| 100 km/h | 98.7 km/h |
| 120 km/h | 118.4 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 255 mm tire has a manufacturer-specified rim width range.
245/75R17 dimensions · 255/65R18 dimensions · Open in calculator