255/40R18 vs 235/50R17
235/50R17 is a safe swap for 255/40R18 by the usual ±3% rule: the diameter differs by +0.85%.
Speedometer and odometer
With 235/50R17 on a car calibrated for 255/40R18, the speedometer reads 59.5 mph when you're really doing 60, and the odometer under-counts by 0.8%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.3 mph |
| 45 mph | 45.4 mph |
| 60 mph | 60.5 mph |
| 70 mph | 70.6 mph |
| 80 mph | 80.7 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 235/50R17?
235/50R17 is a safe swap for 255/40R18 by the usual ±3% rule: the diameter differs by +0.85%.
What's the difference between 255/40R18 and 235/50R17?
235/50R17 is 0.22" taller and 20 mm narrower; the sidewall is +0.61". At an indicated 60 mph you'd actually be doing 60.5 mph.
Will 235/50R17 fit on 255/40R18 wheels?
No — 255/40R18 needs a 18" wheel and 235/50R17 a 17" wheel. This is a plus/minus-size swap that changes wheels too.