235/50R18 vs 255/40R19
Switching from 235/50R18 to 255/40R19 decreases the overall diameter by 5.6 mm (0.8%). That is within the usual ±3% guideline.
| 235/50R18 | 255/40R19 | Difference (mm / revs) | |
|---|---|---|---|
| Section width | 235 mm | 255 mm | +20.0 |
| Sidewall height | 117.5 mm | 102.0 mm | -15.5 |
| Rim diameter | 457.2 mm | 482.6 mm | +25.4 |
| Overall diameter | 692.2 mm | 686.6 mm | -5.6 |
| Circumference | 2,174.6 mm | 2,157.0 mm | -17.6 |
| Revolutions | 460 per km | 464 per km | +4.0 |
Speedometer effect
With 255/40R19 fitted on a car calibrated for 235/50R18, 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 | 119.0 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 255 mm tire has a manufacturer-specified rim width range.
235/50R18 dimensions · 255/40R19 dimensions · Open in calculator