285/30R20 vs 255/40R20
255/40R20 is outside the ±3% rule as a replacement for 285/30R20: the diameter differs by +4.86%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/40R20 on a car calibrated for 285/30R20, the speedometer reads 57.2 mph when you're really doing 60, and the odometer under-counts by 4.9%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.5 mph |
| 45 mph | 47.2 mph |
| 60 mph | 62.9 mph |
| 70 mph | 73.4 mph |
| 80 mph | 83.9 mph |
Other sizes close to 285/30R20
- 285/30R20 vs 245/40R19-0.06%
- 285/30R20 vs 245/40R20+3.68%
- 285/30R20 vs 275/35R19-0.57%
- 285/30R20 vs 245/35R20+0.07%
- 285/30R20 vs 255/35R20+1.10%
- 285/30R20 vs 275/35R20+3.17%
- 285/30R20 vs 225/45R19+0.90%
- 285/30R20 vs 285/35R19+0.46%
- 285/30R20 vs 285/35R20+4.20%
- 285/30R20 vs 275/30R20-0.88%
- 285/30R20 vs 265/35R20+2.14%
- 285/30R20 vs 235/35R20-0.96%
Questions
Can I replace 285/30R20 with 255/40R20?
255/40R20 is outside the ±3% rule as a replacement for 285/30R20: the diameter differs by +4.86%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 285/30R20 and 255/40R20?
255/40R20 is 1.30" taller and 30 mm narrower; the sidewall is +0.65". At an indicated 60 mph you'd actually be doing 62.9 mph.
Will 255/40R20 fit on 285/30R20 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.