255/40R18 vs 255/45R18
255/45R18 is outside the ±3% rule as a replacement for 255/40R18: the diameter differs by +3.86%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/45R18 on a car calibrated for 255/40R18, the speedometer reads 57.8 mph when you're really doing 60, and the odometer under-counts by 3.9%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.2 mph |
| 45 mph | 46.7 mph |
| 60 mph | 62.3 mph |
| 70 mph | 72.7 mph |
| 80 mph | 83.1 mph |
Other sizes close to 255/40R18
- 255/40R18 vs 225/45R18-0.23%
- 255/40R18 vs 245/40R18-1.21%
- 255/40R18 vs 225/40R18-3.63%
- 255/40R18 vs 225/50R17-0.67%
- 255/40R18 vs 245/45R18+2.50%
- 255/40R18 vs 235/50R18+4.69%
- 255/40R18 vs 235/45R18+1.13%
- 255/40R18 vs 235/40R18-2.42%
- 255/40R18 vs 225/50R18+3.18%
- 255/40R18 vs 255/35R19-0.02%
- 255/40R18 vs 255/35R18-3.86%
- 255/40R18 vs 225/40R19+0.21%
Questions
Can I replace 255/40R18 with 255/45R18?
255/45R18 is outside the ±3% rule as a replacement for 255/40R18: the diameter differs by +3.86%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 255/40R18 and 255/45R18?
255/45R18 is 1.00" taller and 0 mm wider; the sidewall is +0.50". At an indicated 60 mph you'd actually be doing 62.3 mph.
Will 255/45R18 fit on 255/40R18 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.