205/55R17 vs 255/40R18
255/40R18 is a safe swap for 205/55R17 by the usual ±3% rule: the diameter differs by +0.59%.
Speedometer and odometer
With 255/40R18 on a car calibrated for 205/55R17, the speedometer reads 59.6 mph when you're really doing 60, and the odometer under-counts by 0.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.3 mph |
| 60 mph | 60.4 mph |
| 70 mph | 70.4 mph |
| 80 mph | 80.5 mph |
Other sizes close to 205/55R17
- 205/55R17 vs 225/45R17-3.50%
- 205/55R17 vs 225/45R18+0.37%
- 205/55R17 vs 245/40R18-0.62%
- 205/55R17 vs 225/50R17-0.08%
- 205/55R17 vs 225/55R17+3.35%
- 205/55R17 vs 215/45R17-4.87%
- 205/55R17 vs 205/50R17-3.12%
- 205/55R17 vs 215/55R17+1.67%
- 205/55R17 vs 215/50R17-1.60%
- 205/55R17 vs 235/45R17-2.13%
- 205/55R17 vs 245/45R17-0.76%
- 205/55R17 vs 205/60R16-0.75%
Questions
Can I replace 205/55R17 with 255/40R18?
255/40R18 is a safe swap for 205/55R17 by the usual ±3% rule: the diameter differs by +0.59%.
What's the difference between 205/55R17 and 255/40R18?
255/40R18 is 0.15" taller and 50 mm wider; the sidewall is -0.42". At an indicated 60 mph you'd actually be doing 60.4 mph.
Will 255/40R18 fit on 205/55R17 wheels?
No — 205/55R17 needs a 17" wheel and 255/40R18 a 18" wheel. This is a plus/minus-size swap that changes wheels too.