215/45R18 vs 255/40R18
255/40R18 is a safe swap for 215/45R18 by the usual ±3% rule: the diameter differs by +1.61%.
Speedometer and odometer
With 255/40R18 on a car calibrated for 215/45R18, the speedometer reads 59.0 mph when you're really doing 60, and the odometer under-counts by 1.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.5 mph |
| 45 mph | 45.7 mph |
| 60 mph | 61.0 mph |
| 70 mph | 71.1 mph |
| 80 mph | 81.3 mph |
Other sizes close to 215/45R18
- 215/45R18 vs 245/40R18+0.38%
- 215/45R18 vs 225/45R18+1.38%
- 215/45R18 vs 225/40R18-2.07%
- 215/45R18 vs 225/50R17+0.94%
- 215/45R18 vs 245/45R18+4.15%
- 215/45R18 vs 235/45R18+2.77%
- 215/45R18 vs 235/40R18-0.85%
- 215/45R18 vs 225/50R18+4.84%
- 215/45R18 vs 215/50R17-0.60%
- 215/45R18 vs 245/45R17+0.25%
- 215/45R18 vs 255/35R18-2.31%
- 215/45R18 vs 245/35R19+0.52%
Questions
Can I replace 215/45R18 with 255/40R18?
255/40R18 is a safe swap for 215/45R18 by the usual ±3% rule: the diameter differs by +1.61%.
What's the difference between 215/45R18 and 255/40R18?
255/40R18 is 0.41" taller and 40 mm wider; the sidewall is +0.21". At an indicated 60 mph you'd actually be doing 61.0 mph.
Will 255/40R18 fit on 215/45R18 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.