255/40R21 vs 315/35R21
315/35R21 is a safe swap for 255/40R21 by the usual ±3% rule: the diameter differs by +2.24%.
Speedometer and odometer
With 315/35R21 on a car calibrated for 255/40R21, 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 | 30.7 mph |
| 45 mph | 46.0 mph |
| 60 mph | 61.3 mph |
| 70 mph | 71.6 mph |
| 80 mph | 81.8 mph |
Other sizes close to 255/40R21
- 255/40R21 vs 255/45R20+0.01%
- 255/40R21 vs 265/40R21+1.08%
- 255/40R21 vs 275/35R21-1.56%
- 255/40R21 vs 295/35R21+0.34%
- 255/40R21 vs 265/35R21-2.51%
- 255/40R21 vs 265/35R22+0.94%
- 255/40R21 vs 295/40R20+0.90%
- 255/40R21 vs 295/40R21+4.34%
- 255/40R21 vs 245/35R21-4.41%
- 255/40R21 vs 285/40R21+3.25%
- 255/40R21 vs 265/45R21+4.68%
- 255/40R21 vs 285/40R20-0.19%
Questions
Can I replace 255/40R21 with 315/35R21?
315/35R21 is a safe swap for 255/40R21 by the usual ±3% rule: the diameter differs by +2.24%.
What's the difference between 255/40R21 and 315/35R21?
315/35R21 is 0.65" taller and 60 mm wider; the sidewall is +0.32". At an indicated 60 mph you'd actually be doing 61.3 mph.
Will 315/35R21 fit on 255/40R21 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.