315/30R21 vs 245/45R20
245/45R20 is a safe swap for 315/30R21 by the usual ±3% rule: the diameter differs by +0.84%.
Speedometer and odometer
With 245/45R20 on a car calibrated for 315/30R21, the speedometer reads 59.5 mph when you're really doing 60, and the odometer under-counts by 0.8%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.3 mph |
| 45 mph | 45.4 mph |
| 60 mph | 60.5 mph |
| 70 mph | 70.6 mph |
| 80 mph | 80.7 mph |
Other sizes close to 315/30R21
- 315/30R21 vs 275/40R20+0.78%
- 315/30R21 vs 265/40R21+3.18%
- 315/30R21 vs 315/35R20+0.84%
- 315/30R21 vs 275/35R21+0.48%
- 315/30R21 vs 295/35R21+2.42%
- 315/30R21 vs 255/40R21+2.08%
- 315/30R21 vs 265/35R21-0.48%
- 315/30R21 vs 245/35R21-2.42%
- 315/30R21 vs 305/35R20-0.12%
- 315/30R21 vs 325/30R21+0.83%
- 315/30R21 vs 275/40R21+4.29%
- 315/30R21 vs 235/45R20-0.40%
Questions
Can I replace 315/30R21 with 245/45R20?
245/45R20 is a safe swap for 315/30R21 by the usual ±3% rule: the diameter differs by +0.84%.
What's the difference between 315/30R21 and 245/45R20?
245/45R20 is 0.24" taller and 70 mm narrower; the sidewall is +0.62". At an indicated 60 mph you'd actually be doing 60.5 mph.
Will 245/45R20 fit on 315/30R21 wheels?
No — 315/30R21 needs a 21" wheel and 245/45R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.