235/40R18 vs 315/30R18
315/30R18 is a safe swap for 235/40R18 by the usual ±3% rule: the diameter differs by +0.15%.
Speedometer and odometer
With 315/30R18 on a car calibrated for 235/40R18, the speedometer reads 59.9 mph when you're really doing 60, and the odometer under-counts by 0.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.0 mph |
| 45 mph | 45.1 mph |
| 60 mph | 60.1 mph |
| 70 mph | 70.1 mph |
| 80 mph | 80.1 mph |
Other sizes close to 235/40R18
- 235/40R18 vs 245/40R18+1.24%
- 235/40R18 vs 225/45R18+2.25%
- 235/40R18 vs 225/40R18-1.24%
- 235/40R18 vs 235/45R18+3.64%
- 235/40R18 vs 215/50R17+0.25%
- 235/40R18 vs 235/45R17-0.29%
- 235/40R18 vs 255/35R18-1.47%
- 235/40R18 vs 255/40R18+2.48%
- 235/40R18 vs 235/35R19+0.29%
- 235/40R18 vs 275/35R18+0.70%
- 235/40R18 vs 265/35R18-0.39%
- 235/40R18 vs 275/40R18+4.96%
Questions
Can I replace 235/40R18 with 315/30R18?
315/30R18 is a safe swap for 235/40R18 by the usual ±3% rule: the diameter differs by +0.15%.
What's the difference between 235/40R18 and 315/30R18?
315/30R18 is 0.04" taller and 80 mm wider; the sidewall is +0.02". At an indicated 60 mph you'd actually be doing 60.1 mph.
Will 315/30R18 fit on 235/40R18 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.