315/35R21 vs 285/40R21
285/40R21 is a safe swap for 315/35R21 by the usual ±3% rule: the diameter differs by +0.99%.
Speedometer and odometer
With 285/40R21 on a car calibrated for 315/35R21, the speedometer reads 59.4 mph when you're really doing 60, and the odometer under-counts by 1.0%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.3 mph |
| 45 mph | 45.4 mph |
| 60 mph | 60.6 mph |
| 70 mph | 70.7 mph |
| 80 mph | 80.8 mph |
Other sizes close to 315/35R21
- 315/35R21 vs 245/50R20-0.12%
- 315/35R21 vs 275/45R20+0.21%
- 315/35R21 vs 265/45R20-0.98%
- 315/35R21 vs 265/40R21-1.13%
- 315/35R21 vs 275/35R21-3.71%
- 315/35R21 vs 295/35R21-1.86%
- 315/35R21 vs 275/45R21+3.58%
- 315/35R21 vs 255/40R21-2.19%
- 315/35R21 vs 265/35R21-4.64%
- 315/35R21 vs 285/35R22+0.58%
- 315/35R21 vs 295/40R21+2.06%
- 315/35R21 vs 285/45R21+4.78%
Questions
Can I replace 315/35R21 with 285/40R21?
285/40R21 is a safe swap for 315/35R21 by the usual ±3% rule: the diameter differs by +0.99%.
What's the difference between 315/35R21 and 285/40R21?
285/40R21 is 0.30" taller and 30 mm narrower; the sidewall is +0.15". At an indicated 60 mph you'd actually be doing 60.6 mph.
Will 285/40R21 fit on 315/35R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 285 mm tire and the fender clearance.