305/25R20 vs 285/30R20
285/30R20 is a safe swap for 305/25R20 by the usual ±3% rule: the diameter differs by +2.80%.
Speedometer and odometer
With 285/30R20 on a car calibrated for 305/25R20, the speedometer reads 58.4 mph when you're really doing 60, and the odometer under-counts by 2.8%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.8 mph |
| 45 mph | 46.3 mph |
| 60 mph | 61.7 mph |
| 70 mph | 72.0 mph |
| 80 mph | 82.2 mph |
Other sizes close to 305/25R20
- 305/25R20 vs 255/35R19+0.09%
- 305/25R20 vs 245/35R20+2.88%
- 305/25R20 vs 255/35R20+3.94%
- 305/25R20 vs 245/35R19-0.97%
- 305/25R20 vs 225/40R19+0.32%
- 305/25R20 vs 275/30R20+1.89%
- 305/25R20 vs 265/35R20+5.00%
- 305/25R20 vs 235/35R20+1.82%
- 305/25R20 vs 295/30R20+3.71%
- 305/25R20 vs 305/30R20+4.62%
- 305/25R20 vs 305/30R19+0.77%
- 305/25R20 vs 295/30R19-0.14%
Questions
Can I replace 305/25R20 with 285/30R20?
285/30R20 is a safe swap for 305/25R20 by the usual ±3% rule: the diameter differs by +2.80%.
What's the difference between 305/25R20 and 285/30R20?
285/30R20 is 0.73" taller and 20 mm narrower; the sidewall is +0.36". At an indicated 60 mph you'd actually be doing 61.7 mph.
Will 285/30R20 fit on 305/25R20 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.