315/40R21 vs 255/55R20
255/55R20 is a safe swap for 315/40R21 by the usual ±3% rule: the diameter differs by +0.39%.
Speedometer and odometer
With 255/55R20 on a car calibrated for 315/40R21, the speedometer reads 59.8 mph when you're really doing 60, and the odometer under-counts by 0.4%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.1 mph |
| 45 mph | 45.2 mph |
| 60 mph | 60.2 mph |
| 70 mph | 70.3 mph |
| 80 mph | 80.3 mph |
Other sizes close to 315/40R21
- 315/40R21 vs 275/50R20-0.31%
- 315/40R21 vs 275/40R22-0.84%
- 315/40R21 vs 275/45R21-0.57%
- 315/40R21 vs 285/40R22+0.18%
- 315/40R21 vs 285/45R21+0.57%
- 315/40R21 vs 295/40R21-2.04%
- 315/40R21 vs 285/50R20+0.97%
- 315/40R21 vs 265/45R21-1.72%
- 315/40R21 vs 285/40R21-3.06%
- 315/40R21 vs 275/40R21-4.07%
- 315/40R21 vs 315/35R21-4.01%
- 315/40R21 vs 325/35R22+0.11%
Questions
Can I replace 315/40R21 with 255/55R20?
255/55R20 is a safe swap for 315/40R21 by the usual ±3% rule: the diameter differs by +0.39%.
What's the difference between 315/40R21 and 255/55R20?
255/55R20 is 0.12" taller and 60 mm narrower; the sidewall is +0.56". At an indicated 60 mph you'd actually be doing 60.2 mph.
Will 255/55R20 fit on 315/40R21 wheels?
No — 315/40R21 needs a 21" wheel and 255/55R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.