305/55R20 vs 285/65R20
285/65R20 is outside the ±3% rule as a replacement for 305/55R20: the diameter differs by +4.15%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 285/65R20 on a car calibrated for 305/55R20, the speedometer reads 57.6 mph when you're really doing 60, and the odometer under-counts by 4.1%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.2 mph |
| 45 mph | 46.9 mph |
| 60 mph | 62.5 mph |
| 70 mph | 72.9 mph |
| 80 mph | 83.3 mph |
Other sizes close to 305/55R20
- 305/55R20 vs 275/55R20-3.91%
- 305/55R20 vs 275/60R20-0.65%
- 305/55R20 vs 275/65R20+2.61%
- 305/55R20 vs 265/60R20-2.07%
- 305/55R20 vs 285/55R20-2.61%
- 305/55R20 vs 285/60R20+0.77%
- 305/55R20 vs 33x12.50R20-0.63%
- 305/55R20 vs 295/60R20+2.19%
- 305/55R20 vs 295/55R20-1.30%
- 305/55R20 vs 245/60R20-4.92%
- 305/55R20 vs 305/50R20-3.62%
- 305/55R20 vs 255/60R20-3.50%
Questions
Can I replace 305/55R20 with 285/65R20?
285/65R20 is outside the ±3% rule as a replacement for 305/55R20: the diameter differs by +4.15%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 305/55R20 and 285/65R20?
285/65R20 is 1.38" taller and 20 mm narrower; the sidewall is +0.69". At an indicated 60 mph you'd actually be doing 62.5 mph.
Will 285/65R20 fit on 305/55R20 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.