315/30R18 vs 245/40R18
245/40R18 is a safe swap for 315/30R18 by the usual ±3% rule: the diameter differs by +1.08%.
Speedometer and odometer
With 245/40R18 on a car calibrated for 315/30R18, the speedometer reads 59.4 mph when you're really doing 60, and the odometer under-counts by 1.1%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.3 mph |
| 45 mph | 45.5 mph |
| 60 mph | 60.6 mph |
| 70 mph | 70.8 mph |
| 80 mph | 80.9 mph |
Other sizes close to 315/30R18
- 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 255/40R18+2.32%
- 315/30R18 vs 235/35R19+0.14%
- 315/30R18 vs 275/35R18+0.54%
Questions
Can I replace 315/30R18 with 245/40R18?
245/40R18 is a safe swap for 315/30R18 by the usual ±3% rule: the diameter differs by +1.08%.
What's the difference between 315/30R18 and 245/40R18?
245/40R18 is 0.28" taller and 70 mm narrower; the sidewall is +0.14". At an indicated 60 mph you'd actually be doing 60.6 mph.
Will 245/40R18 fit on 315/30R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 245 mm tire and the fender clearance.