225/40R18 vs 255/40R18
255/40R18 is outside the ±3% rule as a replacement for 225/40R18: the diameter differs by +3.77%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/40R18 on a car calibrated for 225/40R18, the speedometer reads 57.8 mph when you're really doing 60, and the odometer under-counts by 3.8%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.1 mph |
| 45 mph | 46.7 mph |
| 60 mph | 62.3 mph |
| 70 mph | 72.6 mph |
| 80 mph | 83.0 mph |
Other sizes close to 225/40R18
- 225/40R18 vs 225/45R17-0.46%
- 225/40R18 vs 245/40R18+2.51%
- 225/40R18 vs 225/45R18+3.53%
- 225/40R18 vs 235/45R18+4.94%
- 225/40R18 vs 205/50R17-0.06%
- 225/40R18 vs 235/40R18+1.26%
- 225/40R18 vs 235/45R17+0.96%
- 225/40R18 vs 255/35R18-0.24%
- 225/40R18 vs 275/35R18+1.96%
- 225/40R18 vs 265/35R18+0.86%
- 225/40R18 vs 255/40R17-0.22%
- 225/40R18 vs 215/45R18+2.12%
Questions
Can I replace 225/40R18 with 255/40R18?
255/40R18 is outside the ±3% rule as a replacement for 225/40R18: the diameter differs by +3.77%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 225/40R18 and 255/40R18?
255/40R18 is 0.94" taller and 30 mm wider; the sidewall is +0.47". At an indicated 60 mph you'd actually be doing 62.3 mph.
Will 255/40R18 fit on 225/40R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.