235/75R15 vs 275/40R20
Switching from 235/75R15 to 275/40R20 decreases the overall diameter by 5.5 mm (0.7%). That is within the usual ±3% guideline.
| 235/75R15 | 275/40R20 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 235 mm | 275 mm | +40.0 |
| Sidewall height | 176.2 mm | 110.0 mm | -66.2 |
| Rim diameter | 381.0 mm | 508.0 mm | +127.0 |
| Overall diameter | 733.5 mm | 728.0 mm | -5.5 |
| Circumference | 2,304.4 mm | 2,287.1 mm | -17.3 |
| Revolutions | 434 per km | 437 per km | +3.0 |
Speedometer effect
With 275/40R20 fitted on a car calibrated for 235/75R15, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.6 km/h |
| 80 km/h | 79.4 km/h |
| 100 km/h | 99.3 km/h |
| 120 km/h | 119.1 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.
235/75R15 dimensions · 275/40R20 dimensions · Open in calculator