275/65R20 vs 275/70R18
Switching from 275/65R20 to 275/70R18 decreases the overall diameter by 23.3 mm (2.7%). That is within the usual ±3% guideline.
| 275/65R20 | 275/70R18 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 275 mm | 275 mm | +0.0 |
| Sidewall height | 178.8 mm | 192.5 mm | +13.7 |
| Rim diameter | 508.0 mm | 457.2 mm | -50.8 |
| Overall diameter | 865.5 mm | 842.2 mm | -23.3 |
| Circumference | 2,719.0 mm | 2,645.8 mm | -73.2 |
| Revolutions | 368 per km | 378 per km | +10.0 |
Speedometer effect
With 275/70R18 fitted on a car calibrated for 275/65R20, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 48.7 km/h |
| 80 km/h | 77.8 km/h |
| 100 km/h | 97.3 km/h |
| 120 km/h | 116.8 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 275 mm tire has a manufacturer-specified rim width range.
275/65R20 dimensions · 275/70R18 dimensions · Open in calculator