315/40R21 vs 275/50R21
275/50R21 is a safe swap for 315/40R21 by the usual ±3% rule: the diameter differs by +2.93%.
Speedometer and odometer
With 275/50R21 on a car calibrated for 315/40R21, the speedometer reads 58.3 mph when you're really doing 60, and the odometer under-counts by 2.9%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.9 mph |
| 45 mph | 46.3 mph |
| 60 mph | 61.8 mph |
| 70 mph | 72.0 mph |
| 80 mph | 82.3 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 275/50R21?
275/50R21 is a safe swap for 315/40R21 by the usual ±3% rule: the diameter differs by +2.93%.
What's the difference between 315/40R21 and 275/50R21?
275/50R21 is 0.91" taller and 40 mm narrower; the sidewall is +0.45". At an indicated 60 mph you'd actually be doing 61.8 mph.
Will 275/50R21 fit on 315/40R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 275 mm tire and the fender clearance.