215/40R18 vs 315/30R18
315/30R18 is a safe swap for 215/40R18 by the usual ±3% rule: the diameter differs by +2.70%.
Speedometer and odometer
With 315/30R18 on a car calibrated for 215/40R18, the speedometer reads 58.4 mph when you're really doing 60, and the odometer under-counts by 2.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.8 mph |
| 45 mph | 46.2 mph |
| 60 mph | 61.6 mph |
| 70 mph | 71.9 mph |
| 80 mph | 82.2 mph |
Other sizes close to 215/40R18
- 215/40R18 vs 225/45R17+0.81%
- 215/40R18 vs 245/40R18+3.81%
- 215/40R18 vs 225/45R18+4.85%
- 215/40R18 vs 225/40R18+1.27%
- 215/40R18 vs 215/45R17-0.62%
- 215/40R18 vs 235/40R18+2.54%
- 215/40R18 vs 255/35R18+1.03%
- 215/40R18 vs 245/40R17-0.22%
- 215/40R18 vs 275/35R18+3.26%
- 215/40R18 vs 265/35R18+2.15%
- 215/40R18 vs 215/45R18+3.42%
- 215/40R18 vs 245/35R18-0.08%
Questions
Can I replace 215/40R18 with 315/30R18?
315/30R18 is a safe swap for 215/40R18 by the usual ±3% rule: the diameter differs by +2.70%.
What's the difference between 215/40R18 and 315/30R18?
315/30R18 is 0.67" taller and 100 mm wider; the sidewall is +0.33". At an indicated 60 mph you'd actually be doing 61.6 mph.
Will 315/30R18 fit on 215/40R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 315 mm tire and the fender clearance.