315/30R18 vs 255/40R18
255/40R18 is a safe swap for 315/30R18 by the usual ±3% rule: the diameter differs by +2.32%.
Speedometer and odometer
With 255/40R18 on a car calibrated for 315/30R18, the speedometer reads 58.6 mph when you're really doing 60, and the odometer under-counts by 2.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.7 mph |
| 45 mph | 46.0 mph |
| 60 mph | 61.4 mph |
| 70 mph | 71.6 mph |
| 80 mph | 81.9 mph |
Other sizes close to 315/30R18
- 315/30R18 vs 245/40R18+1.08%
- 315/30R18 vs 225/45R18+2.09%
- 315/30R18 vs 225/40R18-1.39%
- 315/30R18 vs 245/45R18+4.87%
- 315/30R18 vs 235/45R18+3.48%
- 315/30R18 vs 235/40R18-0.15%
- 315/30R18 vs 215/50R17+0.09%
- 315/30R18 vs 235/45R17-0.45%
- 315/30R18 vs 245/45R17+0.94%
- 315/30R18 vs 255/35R18-1.62%
- 315/30R18 vs 235/35R19+0.14%
- 315/30R18 vs 275/35R18+0.54%
Questions
Can I replace 315/30R18 with 255/40R18?
255/40R18 is a safe swap for 315/30R18 by the usual ±3% rule: the diameter differs by +2.32%.
What's the difference between 315/30R18 and 255/40R18?
255/40R18 is 0.59" taller and 60 mm narrower; the sidewall is +0.30". At an indicated 60 mph you'd actually be doing 61.4 mph.
Will 255/40R18 fit on 315/30R18 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.