205/40R18 vs 315/30R18
315/30R18 is outside the ±3% rule as a replacement for 205/40R18: the diameter differs by +4.02%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 315/30R18 on a car calibrated for 205/40R18, the speedometer reads 57.7 mph when you're really doing 60, and the odometer under-counts by 4.0%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.2 mph |
| 45 mph | 46.8 mph |
| 60 mph | 62.4 mph |
| 70 mph | 72.8 mph |
| 80 mph | 83.2 mph |
Other sizes close to 205/40R18
- 205/40R18 vs 225/40R18+2.58%
- 205/40R18 vs 215/45R17+0.66%
- 205/40R18 vs 235/40R18+3.86%
- 205/40R18 vs 255/35R18+2.33%
- 205/40R18 vs 205/45R17-0.79%
- 205/40R18 vs 275/35R18+4.59%
- 205/40R18 vs 265/35R18+3.46%
- 205/40R18 vs 215/45R18+4.75%
- 205/40R18 vs 245/35R18+1.21%
- 205/40R18 vs 215/40R18+1.29%
- 205/40R18 vs 295/30R18+2.09%
- 205/40R18 vs 285/30R18+1.13%
Questions
Can I replace 205/40R18 with 315/30R18?
315/30R18 is outside the ±3% rule as a replacement for 205/40R18: the diameter differs by +4.02%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 205/40R18 and 315/30R18?
315/30R18 is 0.98" taller and 110 mm wider; the sidewall is +0.49". At an indicated 60 mph you'd actually be doing 62.4 mph.
Will 315/30R18 fit on 205/40R18 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.