275/60R20 vs 285/70R17
Switching from 275/60R20 to 285/70R17 decreases the overall diameter by 7.2 mm (0.9%). That is within the usual ±3% guideline.
| 275/60R20 | 285/70R17 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 275 mm | 285 mm | +10.0 |
| Sidewall height | 165.0 mm | 199.5 mm | +34.5 |
| Rim diameter | 508.0 mm | 431.8 mm | -76.2 |
| Overall diameter | 838.0 mm | 830.8 mm | -7.2 |
| Circumference | 2,632.7 mm | 2,610.0 mm | -22.7 |
| Revolutions | 380 per km | 383 per km | +3.0 |
Speedometer effect
With 285/70R17 fitted on a car calibrated for 275/60R20, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.6 km/h |
| 80 km/h | 79.3 km/h |
| 100 km/h | 99.1 km/h |
| 120 km/h | 119.0 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.
275/60R20 dimensions · 285/70R17 dimensions · Open in calculator