315/30R20 vs 255/40R20
255/40R20 is a safe swap for 315/30R20 by the usual ±3% rule: the diameter differs by +2.15%.
Speedometer and odometer
With 255/40R20 on a car calibrated for 315/30R20, the speedometer reads 58.7 mph when you're really doing 60, and the odometer under-counts by 2.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.6 mph |
| 45 mph | 46.0 mph |
| 60 mph | 61.3 mph |
| 70 mph | 71.5 mph |
| 80 mph | 81.7 mph |
Other sizes close to 315/30R20
- 315/30R20 vs 245/45R19+0.88%
- 315/30R20 vs 245/45R20+4.52%
- 315/30R20 vs 275/40R20+4.45%
- 315/30R20 vs 245/40R20+1.00%
- 315/30R20 vs 275/40R19+0.80%
- 315/30R20 vs 245/35R20-2.51%
- 315/30R20 vs 255/35R20-1.51%
- 315/30R20 vs 275/35R20+0.50%
- 315/30R20 vs 235/45R19-0.42%
- 315/30R20 vs 285/35R20+1.51%
- 315/30R20 vs 315/35R20+4.52%
- 315/30R20 vs 285/30R20-2.58%
Questions
Can I replace 315/30R20 with 255/40R20?
255/40R20 is a safe swap for 315/30R20 by the usual ±3% rule: the diameter differs by +2.15%.
What's the difference between 315/30R20 and 255/40R20?
255/40R20 is 0.59" taller and 60 mm narrower; the sidewall is +0.30". At an indicated 60 mph you'd actually be doing 61.3 mph.
Will 255/40R20 fit on 315/30R20 wheels?
Both use a 20" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.