275/80R24.5 vs 285/75R24.5
Going from one to the other changes the overall height by -1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/80R24.5 | 285/75R24.5 | Change |
|---|---|---|---|
| Overall height | 41.8" | 41.3" | -1.2 % |
| Sidewall | 8.66" | 8.42" | -2.8 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 131.4" | 129.8" | -1.2 % |
| Revs per mile | 482 | 488 | +1.2 % |
| Axle height | 20.91" | 20.67" | -6.3 mm |
| Approved rim width | 7.5–10" | 8–10.5" | overlap 8–10" |
Both are approved on a rim 8" to 10" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 275/80R24.5. Fit 285/75R24.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 88.9 mph | -1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,882 on the road, so it over-counts by 118. At a true 70 mph the first turns 563 times a minute, the second 569.