255/40R18 vs 235/50R18
235/50R18 is outside the ±3% rule as a replacement for 255/40R18: the diameter differs by +4.69%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 235/50R18 on a car calibrated for 255/40R18, the speedometer reads 57.3 mph when you're really doing 60, and the odometer under-counts by 4.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.4 mph |
| 45 mph | 47.1 mph |
| 60 mph | 62.8 mph |
| 70 mph | 73.3 mph |
| 80 mph | 83.8 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 235/45R18+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%
- 255/40R18 vs 275/35R18-1.74%
Questions
Can I replace 255/40R18 with 235/50R18?
235/50R18 is outside the ±3% rule as a replacement for 255/40R18: the diameter differs by +4.69%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 255/40R18 and 235/50R18?
235/50R18 is 1.22" taller and 20 mm narrower; the sidewall is +0.61". At an indicated 60 mph you'd actually be doing 62.8 mph.
Will 235/50R18 fit on 255/40R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 235 mm tire and the fender clearance.