255/45R21 vs 315/40R21
315/40R21 is a safe swap for 255/45R21 by the usual ±3% rule: the diameter differs by +2.95%.
Speedometer and odometer
With 315/40R21 on a car calibrated for 255/45R21, 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 | 72.1 mph |
| 80 mph | 82.4 mph |
Other sizes close to 255/45R21
- 255/45R21 vs 255/50R20+0.01%
- 255/45R21 vs 275/45R20-0.97%
- 255/45R21 vs 235/55R20+0.47%
- 255/45R21 vs 265/40R21-2.29%
- 255/45R21 vs 275/35R21-4.85%
- 255/45R21 vs 295/35R21-3.01%
- 255/45R21 vs 275/45R21+2.36%
- 255/45R21 vs 255/40R21-3.34%
- 255/45R21 vs 285/35R22-0.60%
- 255/45R21 vs 295/40R21+0.85%
- 255/45R21 vs 285/45R21+3.54%
- 255/45R21 vs 285/40R21-0.20%
Questions
Can I replace 255/45R21 with 315/40R21?
315/40R21 is a safe swap for 255/45R21 by the usual ±3% rule: the diameter differs by +2.95%.
What's the difference between 255/45R21 and 315/40R21?
315/40R21 is 0.89" taller and 60 mm wider; the sidewall is +0.44". At an indicated 60 mph you'd actually be doing 61.8 mph.
Will 315/40R21 fit on 255/45R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 315 mm tire and the fender clearance.