225/40R18 vs 315/30R18
315/30R18 is a safe swap for 225/40R18 by the usual ±3% rule: the diameter differs by +1.41%.
Speedometer and odometer
With 315/30R18 on a car calibrated for 225/40R18, the speedometer reads 59.2 mph when you're really doing 60, and the odometer under-counts by 1.4%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.4 mph |
| 45 mph | 45.6 mph |
| 60 mph | 60.8 mph |
| 70 mph | 71.0 mph |
| 80 mph | 81.1 mph |
Other sizes close to 225/40R18
- 225/40R18 vs 225/45R17-0.46%
- 225/40R18 vs 245/40R18+2.51%
- 225/40R18 vs 225/45R18+3.53%
- 225/40R18 vs 235/45R18+4.94%
- 225/40R18 vs 205/50R17-0.06%
- 225/40R18 vs 235/40R18+1.26%
- 225/40R18 vs 235/45R17+0.96%
- 225/40R18 vs 255/35R18-0.24%
- 225/40R18 vs 255/40R18+3.77%
- 225/40R18 vs 275/35R18+1.96%
- 225/40R18 vs 265/35R18+0.86%
- 225/40R18 vs 255/40R17-0.22%
Questions
Can I replace 225/40R18 with 315/30R18?
315/30R18 is a safe swap for 225/40R18 by the usual ±3% rule: the diameter differs by +1.41%.
What's the difference between 225/40R18 and 315/30R18?
315/30R18 is 0.35" taller and 90 mm wider; the sidewall is +0.18". At an indicated 60 mph you'd actually be doing 60.8 mph.
Will 315/30R18 fit on 225/40R18 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.