305/25R21 vs 285/30R21
Going from one to the other changes the overall height by +2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/25R21 | 285/30R21 | Change |
|---|---|---|---|
| Overall height | 27.0" | 27.7" | +2.7 % |
| Sidewall | 3.00" | 3.37" | +12.1 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 84.8" | 87.1" | +2.7 % |
| Revs per mile | 747 | 727 | -2.7 % |
| Axle height | 13.50" | 13.87" | +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 305/25R21. Fit 285/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,270 on the road, so it under-counts by 270. At a true 70 mph the first turns 871 times a minute, the second 848.