275/70R18 vs 285/70R17
Switching from 275/70R18 to 285/70R17 decreases the overall diameter by 11.4 mm (1.4%). That is within the usual ±3% guideline.
| 275/70R18 | 285/70R17 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 275 mm | 285 mm | +10.0 |
| Sidewall height | 192.5 mm | 199.5 mm | +7.0 |
| Rim diameter | 457.2 mm | 431.8 mm | -25.4 |
| Overall diameter | 842.2 mm | 830.8 mm | -11.4 |
| Circumference | 2,645.8 mm | 2,610.0 mm | -35.8 |
| Revolutions | 378 per km | 383 per km | +5.0 |
Speedometer effect
With 285/70R17 fitted on a car calibrated for 275/70R18, 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 | 98.6 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 285 mm tire has a manufacturer-specified rim width range.
275/70R18 dimensions · 285/70R17 dimensions · Open in calculator