255/50R21 vs 285/50R20
285/50R20 is a safe swap for 255/50R21 by the usual ±3% rule: the diameter differs by +0.58%.
Speedometer and odometer
With 285/50R20 on a car calibrated for 255/50R21, the speedometer reads 59.7 mph when you're really doing 60, and the odometer under-counts by 0.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.3 mph |
| 60 mph | 60.4 mph |
| 70 mph | 70.4 mph |
| 80 mph | 80.5 mph |
Other sizes close to 255/50R21
- 255/50R21 vs 255/55R20+0.01%
- 255/50R21 vs 275/50R20-0.68%
- 255/50R21 vs 275/45R21-0.95%
- 255/50R21 vs 285/40R22-0.20%
- 255/50R21 vs 285/45R21+0.19%
- 255/50R21 vs 295/40R21-2.41%
- 255/50R21 vs 265/45R21-2.09%
- 255/50R21 vs 285/40R21-3.42%
- 255/50R21 vs 275/40R21-4.44%
- 255/50R21 vs 315/35R21-4.38%
- 255/50R21 vs 315/40R21-0.38%
- 255/50R21 vs 325/35R22-0.27%
Questions
Can I replace 255/50R21 with 285/50R20?
285/50R20 is a safe swap for 255/50R21 by the usual ±3% rule: the diameter differs by +0.58%.
What's the difference between 255/50R21 and 285/50R20?
285/50R20 is 0.18" taller and 30 mm wider; the sidewall is +0.59". At an indicated 60 mph you'd actually be doing 60.4 mph.
Will 285/50R20 fit on 255/50R21 wheels?
No — 255/50R21 needs a 21" wheel and 285/50R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.