305/35R20 vs 285/35R20
Going from one to the other changes the overall height by -1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/35R20 | 285/35R20 | Change |
|---|---|---|---|
| Overall height | 28.4" | 27.9" | -1.9 % |
| Sidewall | 4.20" | 3.93" | -6.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 89.2" | 87.5" | -1.9 % |
| Revs per mile | 710 | 724 | +1.9 % |
| Axle height | 14.20" | 13.93" | -7.0 mm |
| Approved rim width | 8.5–11" | 8–10.5" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/35R20. Fit 285/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,806 on the road, so it over-counts by 194. At a true 70 mph the first turns 828 times a minute, the second 845.