285/60R20 vs 265/60R20
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/60R20 | 265/60R20 | Change |
|---|---|---|---|
| Overall height | 33.5" | 32.5" | -2.8 % |
| Sidewall | 6.73" | 6.26" | -7.0 % |
| Section width | 285 mm | 265 mm | -7.0 % |
| Circumference | 105.1" | 102.2" | -2.8 % |
| Revs per mile | 603 | 620 | +2.8 % |
| Axle height | 16.73" | 16.26" | -12.0 mm |
| Approved rim width | 8–10.5" | 7.5–9.5" | 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 285/60R20. Fit 265/60R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,718 on the road, so it over-counts by 282. At a true 70 mph the first turns 703 times a minute, the second 724.