255/40R21 vs 285/40R21
285/40R21 is outside the ±3% rule as a replacement for 255/40R21: the diameter differs by +3.25%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 285/40R21 on a car calibrated for 255/40R21, the speedometer reads 58.1 mph when you're really doing 60, and the odometer under-counts by 3.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.0 mph |
| 45 mph | 46.5 mph |
| 60 mph | 62.0 mph |
| 70 mph | 72.3 mph |
| 80 mph | 82.6 mph |
Other sizes close to 255/40R21
- 255/40R21 vs 255/45R20+0.01%
- 255/40R21 vs 265/40R21+1.08%
- 255/40R21 vs 275/35R21-1.56%
- 255/40R21 vs 295/35R21+0.34%
- 255/40R21 vs 265/35R21-2.51%
- 255/40R21 vs 265/35R22+0.94%
- 255/40R21 vs 295/40R20+0.90%
- 255/40R21 vs 295/40R21+4.34%
- 255/40R21 vs 245/35R21-4.41%
- 255/40R21 vs 265/45R21+4.68%
- 255/40R21 vs 285/40R20-0.19%
- 255/40R21 vs 325/30R21-1.22%
Questions
Can I replace 255/40R21 with 285/40R21?
285/40R21 is outside the ±3% rule as a replacement for 255/40R21: the diameter differs by +3.25%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 255/40R21 and 285/40R21?
285/40R21 is 0.94" taller and 30 mm wider; the sidewall is +0.47". At an indicated 60 mph you'd actually be doing 62.0 mph.
Will 285/40R21 fit on 255/40R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 285 mm tire and the fender clearance.