245/45R20 vs 275/40R20
Switching from 245/45R20 to 275/40R20 decreases the overall diameter by 0.5 mm (0.1%). That is within the usual ±3% guideline.
| 245/45R20 | 275/40R20 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 245 mm | 275 mm | +30.0 |
| Sidewall height | 110.2 mm | 110.0 mm | -0.2 |
| Rim diameter | 508.0 mm | 508.0 mm | +0.0 |
| Overall diameter | 728.5 mm | 728.0 mm | -0.5 |
| Circumference | 2,288.7 mm | 2,287.1 mm | -1.6 |
| Revolutions | 437 per km | 437 per km | +0.0 |
Speedometer effect
With 275/40R20 fitted on a car calibrated for 245/45R20, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 50.0 km/h |
| 80 km/h | 79.9 km/h |
| 100 km/h | 99.9 km/h |
| 120 km/h | 119.9 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.
245/45R20 dimensions · 275/40R20 dimensions · Open in calculator