305/40R20 vs 285/45R20
285/45R20 is a safe swap for 305/40R20 by the usual ±3% rule: the diameter differs by +1.66%.
Speedometer and odometer
With 285/45R20 on a car calibrated for 305/40R20, the speedometer reads 59.0 mph when you're really doing 60, and the odometer under-counts by 1.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.5 mph |
| 45 mph | 45.7 mph |
| 60 mph | 61.0 mph |
| 70 mph | 71.2 mph |
| 80 mph | 81.3 mph |
Other sizes close to 305/40R20
- 305/40R20 vs 245/45R20-3.12%
- 305/40R20 vs 255/45R20-1.93%
- 305/40R20 vs 255/50R20+1.46%
- 305/40R20 vs 275/40R20-3.19%
- 305/40R20 vs 265/50R20+2.79%
- 305/40R20 vs 245/50R20+0.13%
- 305/40R20 vs 275/45R20+0.47%
- 305/40R20 vs 255/55R20+4.85%
- 305/40R20 vs 235/55R20+1.93%
- 305/40R20 vs 245/55R19+0.01%
- 305/40R20 vs 265/45R20-0.73%
- 305/40R20 vs 265/50R19-0.59%
Questions
Can I replace 305/40R20 with 285/45R20?
285/45R20 is a safe swap for 305/40R20 by the usual ±3% rule: the diameter differs by +1.66%.
What's the difference between 305/40R20 and 285/45R20?
285/45R20 is 0.49" taller and 20 mm narrower; the sidewall is +0.25". At an indicated 60 mph you'd actually be doing 61.0 mph.
Will 285/45R20 fit on 305/40R20 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.