285/30R24 vs 265/30R24
Going from one to the other changes the overall height by -1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/30R24 | 265/30R24 | Change |
|---|---|---|---|
| Overall height | 30.7" | 30.3" | -1.5 % |
| Sidewall | 3.37" | 3.13" | -7.0 % |
| Section width | 285 mm | 265 mm | -7.0 % |
| Circumference | 96.5" | 95.1" | -1.5 % |
| Revs per mile | 656 | 666 | +1.5 % |
| Axle height | 15.37" | 15.13" | -6.0 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 285/30R24. Fit 265/30R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.9 mph | -1.1 |
| 90 mph | 88.6 mph | -1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,846 on the road, so it over-counts by 154. At a true 70 mph the first turns 766 times a minute, the second 778.