235/30R20 vs 285/30R20
285/30R20 is outside the ±3% rule as a replacement for 235/30R20: the diameter differs by +4.62%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 285/30R20 on a car calibrated for 235/30R20, the speedometer reads 57.3 mph when you're really doing 60, and the odometer under-counts by 4.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.4 mph |
| 45 mph | 47.1 mph |
| 60 mph | 62.8 mph |
| 70 mph | 73.2 mph |
| 80 mph | 83.7 mph |
Other sizes close to 235/30R20
- 235/30R20 vs 245/35R20+4.70%
- 235/30R20 vs 245/35R19+0.79%
- 235/30R20 vs 235/35R19-0.29%
- 235/30R20 vs 275/30R20+3.70%
- 235/30R20 vs 285/30R19+0.71%
- 235/30R20 vs 235/35R20+3.62%
- 235/30R20 vs 275/30R19-0.22%
- 235/30R20 vs 245/30R20+0.92%
- 235/30R20 vs 265/30R20+2.77%
- 235/30R20 vs 255/30R20+1.85%
- 235/30R20 vs 225/35R20+2.54%
- 235/30R20 vs 335/25R20+4.08%
Questions
Can I replace 235/30R20 with 285/30R20?
285/30R20 is outside the ±3% rule as a replacement for 235/30R20: the diameter differs by +4.62%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 235/30R20 and 285/30R20?
285/30R20 is 1.18" taller and 50 mm wider; the sidewall is +0.59". At an indicated 60 mph you'd actually be doing 62.8 mph.
Will 285/30R20 fit on 235/30R20 wheels?
Both use a 20" rim, so the wheel diameter matches; check the wheel's width range for a 285 mm tire and the fender clearance.