285/40R20 vs 255/50R20
255/50R20 is outside the ±3% rule as a replacement for 285/40R20: the diameter differs by +3.67%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/50R20 on a car calibrated for 285/40R20, the speedometer reads 57.9 mph when you're really doing 60, and the odometer under-counts by 3.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.1 mph |
| 45 mph | 46.7 mph |
| 60 mph | 62.2 mph |
| 70 mph | 72.6 mph |
| 80 mph | 82.9 mph |
Other sizes close to 285/40R20
- 285/40R20 vs 235/55R19+0.69%
- 285/40R20 vs 245/45R20-1.02%
- 285/40R20 vs 255/45R20+0.20%
- 285/40R20 vs 255/50R19+0.22%
- 285/40R20 vs 275/40R20-1.09%
- 285/40R20 vs 245/40R20-4.35%
- 285/40R20 vs 245/50R20+2.31%
- 285/40R20 vs 275/45R20+2.65%
- 285/40R20 vs 225/55R19-0.80%
- 285/40R20 vs 235/55R20+4.14%
- 285/40R20 vs 255/40R20-3.26%
- 285/40R20 vs 275/35R20-4.82%
Questions
Can I replace 285/40R20 with 255/50R20?
255/50R20 is outside the ±3% rule as a replacement for 285/40R20: the diameter differs by +3.67%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 285/40R20 and 255/50R20?
255/50R20 is 1.06" taller and 30 mm narrower; the sidewall is +0.53". At an indicated 60 mph you'd actually be doing 62.2 mph.
Will 255/50R20 fit on 285/40R20 wheels?
Both use a 20" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.