315/40R21 vs 255/50R21
255/50R21 is a safe swap for 315/40R21 by the usual ±3% rule: the diameter differs by +0.38%.
Speedometer and odometer
With 255/50R21 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 255/55R20+0.39%
- 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%
Questions
Can I replace 315/40R21 with 255/50R21?
255/50R21 is a safe swap for 315/40R21 by the usual ±3% rule: the diameter differs by +0.38%.
What's the difference between 315/40R21 and 255/50R21?
255/50R21 is 0.12" taller and 60 mm narrower; the sidewall is +0.06". At an indicated 60 mph you'd actually be doing 60.2 mph.
Will 255/50R21 fit on 315/40R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.