275/45R20 vs 255/45R21
255/45R21 is a safe swap for 275/45R20 by the usual ±3% rule: the diameter differs by +0.98%.
Speedometer and odometer
With 255/45R21 on a car calibrated for 275/45R20, 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 275/45R20
- 275/45R20 vs 245/45R20-3.57%
- 275/45R20 vs 255/45R20-2.38%
- 275/45R20 vs 255/50R20+0.99%
- 275/45R20 vs 275/40R20-3.64%
- 275/45R20 vs 265/50R20+2.32%
- 275/45R20 vs 245/50R20-0.33%
- 275/45R20 vs 255/55R20+4.37%
- 275/45R20 vs 235/55R20+1.46%
- 275/45R20 vs 245/55R19-0.45%
- 275/45R20 vs 265/45R20-1.19%
- 275/45R20 vs 315/35R20-3.57%
- 275/45R20 vs 275/50R20+3.64%
Questions
Can I replace 275/45R20 with 255/45R21?
255/45R21 is a safe swap for 275/45R20 by the usual ±3% rule: the diameter differs by +0.98%.
What's the difference between 275/45R20 and 255/45R21?
255/45R21 is 0.29" taller and 20 mm narrower; the sidewall is -0.35". At an indicated 60 mph you'd actually be doing 60.6 mph.
Will 255/45R21 fit on 275/45R20 wheels?
No — 275/45R20 needs a 20" wheel and 255/45R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.