245/45R21 vs 315/40R21
315/40R21 is outside the ±3% rule as a replacement for 245/45R21: the diameter differs by +4.18%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 315/40R21 on a car calibrated for 245/45R21, the speedometer reads 57.6 mph when you're really doing 60, and the odometer under-counts by 4.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.3 mph |
| 45 mph | 46.9 mph |
| 60 mph | 62.5 mph |
| 70 mph | 72.9 mph |
| 80 mph | 83.3 mph |
Other sizes close to 245/45R21
- 245/45R21 vs 245/50R20-0.12%
- 245/45R21 vs 275/45R20+0.21%
- 245/45R21 vs 265/45R20-0.98%
- 245/45R21 vs 265/40R21-1.13%
- 245/45R21 vs 275/35R21-3.71%
- 245/45R21 vs 295/35R21-1.86%
- 245/45R21 vs 275/45R21+3.58%
- 245/45R21 vs 255/40R21-2.19%
- 245/45R21 vs 265/35R21-4.64%
- 245/45R21 vs 285/35R22+0.58%
- 245/45R21 vs 295/40R21+2.06%
- 245/45R21 vs 285/45R21+4.78%
Questions
Can I replace 245/45R21 with 315/40R21?
315/40R21 is outside the ±3% rule as a replacement for 245/45R21: the diameter differs by +4.18%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 245/45R21 and 315/40R21?
315/40R21 is 1.24" taller and 70 mm wider; the sidewall is +0.62". At an indicated 60 mph you'd actually be doing 62.5 mph.
Will 315/40R21 fit on 245/45R21 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.