305/35R20 vs 285/40R20
285/40R20 is a safe swap for 305/35R20 by the usual ±3% rule: the diameter differs by +2.01%.
Speedometer and odometer
With 285/40R20 on a car calibrated for 305/35R20, the speedometer reads 58.8 mph when you're really doing 60, and the odometer under-counts by 2.0%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.6 mph |
| 45 mph | 45.9 mph |
| 60 mph | 61.2 mph |
| 70 mph | 71.4 mph |
| 80 mph | 81.6 mph |
Other sizes close to 305/35R20
- 305/35R20 vs 245/45R20+0.97%
- 305/35R20 vs 255/45R20+2.22%
- 305/35R20 vs 275/40R20+0.90%
- 305/35R20 vs 245/40R20-2.43%
- 305/35R20 vs 245/50R20+4.37%
- 305/35R20 vs 275/45R20+4.71%
- 305/35R20 vs 255/35R20-4.85%
- 305/35R20 vs 255/40R20-1.32%
- 305/35R20 vs 275/35R20-2.91%
- 305/35R20 vs 235/50R19-0.54%
- 305/35R20 vs 265/45R20+3.47%
- 305/35R20 vs 285/35R20-1.94%
Questions
Can I replace 305/35R20 with 285/40R20?
285/40R20 is a safe swap for 305/35R20 by the usual ±3% rule: the diameter differs by +2.01%.
What's the difference between 305/35R20 and 285/40R20?
285/40R20 is 0.57" taller and 20 mm narrower; the sidewall is +0.29". At an indicated 60 mph you'd actually be doing 61.2 mph.
Will 285/40R20 fit on 305/35R20 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.