235/65R16 vs 245/40R20
Switching from 235/65R16 to 245/40R20 decreases the overall diameter by 7.9 mm (1.1%). That is within the usual ±3% guideline.
| 235/65R16 | 245/40R20 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 235 mm | 245 mm | +10.0 |
| Sidewall height | 152.8 mm | 98.0 mm | -54.8 |
| Rim diameter | 406.4 mm | 508.0 mm | +101.6 |
| Overall diameter | 711.9 mm | 704.0 mm | -7.9 |
| Circumference | 2,236.5 mm | 2,211.7 mm | -24.8 |
| Revolutions | 447 per km | 452 per km | +5.0 |
Speedometer effect
With 245/40R20 fitted on a car calibrated for 235/65R16, the real speed when the speedometer shows:
| Speedometer shows | Actual speed |
|---|---|
| 50 km/h | 49.4 km/h |
| 80 km/h | 79.1 km/h |
| 100 km/h | 98.9 km/h |
| 120 km/h | 118.7 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.
235/65R16 dimensions · 245/40R20 dimensions · Open in calculator