305/70R24.5 vs 285/75R24.5
Going from one to the other changes the overall height by +0.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/70R24.5 | 285/75R24.5 | Change |
|---|---|---|---|
| Overall height | 41.3" | 41.3" | +0.0 % |
| Sidewall | 8.41" | 8.42" | +0.1 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 129.8" | 129.8" | +0.0 % |
| Revs per mile | 488 | 488 | -0.0 % |
| Axle height | 20.66" | 20.67" | +0.3 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 305/70R24.5. Fit 285/75R24.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | +0.0 |
| 50 mph | 50.0 mph | +0.0 |
| 70 mph | 70.0 mph | +0.0 |
| 90 mph | 90.0 mph | +0.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,005 on the road, so it under-counts by 5. At a true 70 mph the first turns 570 times a minute, the second 569.