305/25R24 vs 285/30R24
Going from one to the other changes the overall height by +2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/25R24 | 285/30R24 | Change |
|---|---|---|---|
| Overall height | 30.0" | 30.7" | +2.4 % |
| Sidewall | 3.00" | 3.37" | +12.1 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 94.3" | 96.5" | +2.4 % |
| Revs per mile | 672 | 656 | -2.4 % |
| Axle height | 15.00" | 15.37" | +9.3 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 305/25R24. Fit 285/30R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,243 on the road, so it under-counts by 243. At a true 70 mph the first turns 784 times a minute, the second 766.