305/25R21 vs 325/25R21
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/25R21 | 325/25R21 | Change |
|---|---|---|---|
| Overall height | 27.0" | 27.4" | +1.5 % |
| Sidewall | 3.00" | 3.20" | +6.6 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 84.8" | 86.1" | +1.5 % |
| Revs per mile | 747 | 736 | -1.5 % |
| Axle height | 13.50" | 13.70" | +5.0 mm |
| Approved rim width | 8.5–11" | 9–12" | 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 305/25R21. Fit 325/25R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,146 on the road, so it under-counts by 146. At a true 70 mph the first turns 871 times a minute, the second 859.