225/70R16 vs 235/55R18
Switching from 225/70R16 to 235/55R18 decreases the overall diameter by 5.7 mm (0.8%). That is within the usual ±3% guideline.
| 225/70R16 | 235/55R18 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 225 mm | 235 mm | +10.0 |
| Sidewall height | 157.5 mm | 129.2 mm | -28.3 |
| Rim diameter | 406.4 mm | 457.2 mm | +50.8 |
| Overall diameter | 721.4 mm | 715.7 mm | -5.7 |
| Circumference | 2,266.3 mm | 2,248.4 mm | -17.9 |
| Revolutions | 441 per km | 445 per km | +4.0 |
Speedometer effect
With 235/55R18 fitted on a car calibrated for 225/70R16, 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 | 99.2 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 235 mm tire has a manufacturer-specified rim width range.
225/70R16 dimensions · 235/55R18 dimensions · Open in calculator