285/40R21 vs 255/50R21
255/50R21 is outside the ±3% rule as a replacement for 285/40R21: the diameter differs by +3.55%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/50R21 on a car calibrated for 285/40R21, the speedometer reads 57.9 mph when you're really doing 60, and the odometer under-counts by 3.5%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.1 mph |
| 45 mph | 46.6 mph |
| 60 mph | 62.1 mph |
| 70 mph | 72.5 mph |
| 80 mph | 82.8 mph |
Other sizes close to 285/40R21
- 285/40R21 vs 255/50R20+0.21%
- 285/40R21 vs 275/45R20-0.77%
- 285/40R21 vs 235/55R20+0.67%
- 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%
Questions
Can I replace 285/40R21 with 255/50R21?
255/50R21 is outside the ±3% rule as a replacement for 285/40R21: the diameter differs by +3.55%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 285/40R21 and 255/50R21?
255/50R21 is 1.06" taller and 30 mm narrower; the sidewall is +0.53". At an indicated 60 mph you'd actually be doing 62.1 mph.
Will 255/50R21 fit on 285/40R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.