325/25R21 vs 305/30R21
Going from one to the other changes the overall height by +2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/25R21 | 305/30R21 | Change |
|---|---|---|---|
| Overall height | 27.4" | 28.2" | +2.9 % |
| Sidewall | 3.20" | 3.60" | +12.6 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 86.1" | 88.6" | +2.9 % |
| Revs per mile | 736 | 715 | -2.9 % |
| Axle height | 13.70" | 14.10" | +10.3 mm |
| Approved rim width | 9–12" | 8.5–11" | overlap 9–11" |
Both are approved on a rim 9" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 325/25R21. Fit 305/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.1 mph | +2.1 |
| 90 mph | 92.7 mph | +2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,295 on the road, so it under-counts by 295. At a true 70 mph the first turns 859 times a minute, the second 834.