285/40R21 vs 235/55R20
235/55R20 is a safe swap for 285/40R21 by the usual ±3% rule: the diameter differs by +0.67%.
Speedometer and odometer
With 235/55R20 on a car calibrated for 285/40R21, the speedometer reads 59.6 mph when you're really doing 60, and the odometer under-counts by 0.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.3 mph |
| 60 mph | 60.4 mph |
| 70 mph | 70.5 mph |
| 80 mph | 80.5 mph |
Other sizes close to 285/40R21
- 285/40R21 vs 255/50R20+0.21%
- 285/40R21 vs 275/45R20-0.77%
- 285/40R21 vs 265/40R21-2.10%
- 285/40R21 vs 275/35R21-4.66%
- 285/40R21 vs 295/35R21-2.82%
- 285/40R21 vs 275/45R21+2.56%
- 285/40R21 vs 255/40R21-3.15%
- 285/40R21 vs 285/35R22-0.41%
- 285/40R21 vs 295/40R21+1.05%
- 285/40R21 vs 285/45R21+3.74%
- 285/40R21 vs 265/45R21+1.38%
- 285/40R21 vs 325/30R21-4.33%
Questions
Can I replace 285/40R21 with 235/55R20?
235/55R20 is a safe swap for 285/40R21 by the usual ±3% rule: the diameter differs by +0.67%.
What's the difference between 285/40R21 and 235/55R20?
235/55R20 is 0.20" taller and 50 mm narrower; the sidewall is +0.60". At an indicated 60 mph you'd actually be doing 60.4 mph.
Will 235/55R20 fit on 285/40R21 wheels?
No — 285/40R21 needs a 21" wheel and 235/55R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.