315/30R20 vs 275/30R21
275/30R21 is a safe swap for 315/30R20 by the usual ±3% rule: the diameter differs by +0.20%.
Speedometer and odometer
With 275/30R21 on a car calibrated for 315/30R20, the speedometer reads 59.9 mph when you're really doing 60, and the odometer under-counts by 0.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.1 mph |
| 45 mph | 45.1 mph |
| 60 mph | 60.1 mph |
| 70 mph | 70.1 mph |
| 80 mph | 80.2 mph |
Other sizes close to 315/30R20
- 315/30R20 vs 245/45R19+0.88%
- 315/30R20 vs 245/45R20+4.52%
- 315/30R20 vs 275/40R20+4.45%
- 315/30R20 vs 245/40R20+1.00%
- 315/30R20 vs 275/40R19+0.80%
- 315/30R20 vs 245/35R20-2.51%
- 315/30R20 vs 255/35R20-1.51%
- 315/30R20 vs 275/35R20+0.50%
- 315/30R20 vs 255/40R20+2.15%
- 315/30R20 vs 235/45R19-0.42%
- 315/30R20 vs 285/35R20+1.51%
- 315/30R20 vs 315/35R20+4.52%
Questions
Can I replace 315/30R20 with 275/30R21?
275/30R21 is a safe swap for 315/30R20 by the usual ±3% rule: the diameter differs by +0.20%.
What's the difference between 315/30R20 and 275/30R21?
275/30R21 is 0.06" taller and 40 mm narrower; the sidewall is -0.47". At an indicated 60 mph you'd actually be doing 60.1 mph.
Will 275/30R21 fit on 315/30R20 wheels?
No — 315/30R20 needs a 20" wheel and 275/30R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.