245/45R21 vs 275/45R20
275/45R20 is a safe swap for 245/45R21 by the usual ±3% rule: the diameter differs by +0.21%.
Speedometer and odometer
With 275/45R20 on a car calibrated for 245/45R21, 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 245/45R21
- 245/45R21 vs 245/50R20-0.12%
- 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%
- 245/45R21 vs 285/40R21+0.99%
Questions
Can I replace 245/45R21 with 275/45R20?
275/45R20 is a safe swap for 245/45R21 by the usual ±3% rule: the diameter differs by +0.21%.
What's the difference between 245/45R21 and 275/45R20?
275/45R20 is 0.06" taller and 30 mm wider; the sidewall is +0.53". At an indicated 60 mph you'd actually be doing 60.1 mph.
Will 275/45R20 fit on 245/45R21 wheels?
No — 245/45R21 needs a 21" wheel and 275/45R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.