255/55R20 vs 285/45R21
285/45R21 is a safe swap for 255/55R20 by the usual ±3% rule: the diameter differs by +0.18%.
Speedometer and odometer
With 285/45R21 on a car calibrated for 255/55R20, 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 | 80.1 mph |
Other sizes close to 255/55R20
- 255/55R20 vs 275/55R20+2.79%
- 255/55R20 vs 255/50R20-3.23%
- 255/55R20 vs 265/50R20-1.97%
- 255/55R20 vs 245/50R20-4.50%
- 255/55R20 vs 275/45R20-4.19%
- 255/55R20 vs 235/55R20-2.79%
- 255/55R20 vs 255/60R19+0.01%
- 255/55R20 vs 265/60R20+4.76%
- 255/55R20 vs 285/55R20+4.19%
- 255/55R20 vs 275/50R20-0.70%
- 255/55R20 vs 275/45R21-0.96%
- 255/55R20 vs 285/50R20+0.57%
Questions
Can I replace 255/55R20 with 285/45R21?
285/45R21 is a safe swap for 255/55R20 by the usual ±3% rule: the diameter differs by +0.18%.
What's the difference between 255/55R20 and 285/45R21?
285/45R21 is 0.06" taller and 30 mm wider; the sidewall is -0.47". At an indicated 60 mph you'd actually be doing 60.1 mph.
Will 285/45R21 fit on 255/55R20 wheels?
No — 255/55R20 needs a 20" wheel and 285/45R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.