285/40R20 vs 305/40R20
305/40R20 is a safe swap for 285/40R20 by the usual ±3% rule: the diameter differs by +2.17%.
Speedometer and odometer
With 305/40R20 on a car calibrated for 285/40R20, the speedometer reads 58.7 mph when you're really doing 60, and the odometer under-counts by 2.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.7 mph |
| 45 mph | 46.0 mph |
| 60 mph | 61.3 mph |
| 70 mph | 71.5 mph |
| 80 mph | 81.7 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 255/50R20+3.67%
- 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%
Questions
Can I replace 285/40R20 with 305/40R20?
305/40R20 is a safe swap for 285/40R20 by the usual ±3% rule: the diameter differs by +2.17%.
What's the difference between 285/40R20 and 305/40R20?
305/40R20 is 0.63" taller and 20 mm wider; the sidewall is +0.31". At an indicated 60 mph you'd actually be doing 61.3 mph.
Will 305/40R20 fit on 285/40R20 wheels?
Both use a 20" rim, so the wheel diameter matches; check the wheel's width range for a 305 mm tire and the fender clearance.