305/70R24.5 vs 285/80R24.5
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 | 305/70R24.5 | 285/80R24.5 | Change |
|---|---|---|---|
| Overall height | 41.3" | 42.5" | +2.8 % |
| Sidewall | 8.41" | 8.98" | +6.8 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 129.8" | 133.4" | +2.8 % |
| Revs per mile | 488 | 475 | -2.8 % |
| Axle height | 20.66" | 21.23" | +14.5 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 285/80R24.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.5 mph | +2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,276 on the road, so it under-counts by 276. At a true 70 mph the first turns 570 times a minute, the second 554.