265/35R20 vs 285/30R20
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/35R20 | 285/30R20 | Change |
|---|---|---|---|
| Overall height | 27.3" | 26.7" | -2.1 % |
| Sidewall | 3.65" | 3.37" | -7.8 % |
| Section width | 265 mm | 285 mm | +7.5 % |
| Circumference | 85.8" | 84.0" | -2.1 % |
| Revs per mile | 739 | 754 | +2.1 % |
| Axle height | 13.65" | 13.37" | -7.3 mm |
| Approved rim width | 7.5–9.5" | 8–10.5" | overlap 8–9.5" |
Both are approved on a rim 8" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 265/35R20. Fit 285/30R20 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 | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,791 on the road, so it over-counts by 209. At a true 70 mph the first turns 862 times a minute, the second 880.