215/65R17 vs 245/40R20
Switching from 215/65R17 to 245/40R20 decreases the overall diameter by 7.3 mm (1.0%). That is within the usual ±3% guideline.
| 215/65R17 | 245/40R20 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 215 mm | 245 mm | +30.0 |
| Sidewall height | 139.8 mm | 98.0 mm | -41.8 |
| Rim diameter | 431.8 mm | 508.0 mm | +76.2 |
| Overall diameter | 711.3 mm | 704.0 mm | -7.3 |
| Circumference | 2,234.6 mm | 2,211.7 mm | -22.9 |
| Revolutions | 448 per km | 452 per km | +4.0 |
Speedometer effect
With 245/40R20 fitted on a car calibrated for 215/65R17, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.5 km/h |
| 80 km/h | 79.2 km/h |
| 100 km/h | 99.0 km/h |
| 120 km/h | 118.8 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 245 mm tire has a manufacturer-specified rim width range.
215/65R17 dimensions · 245/40R20 dimensions · Open in calculator