305/30R21 vs 325/30R21
Going from one to the other changes the overall height by +1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/30R21 | 325/30R21 | Change |
|---|---|---|---|
| Overall height | 28.2" | 28.7" | +1.7 % |
| Sidewall | 3.60" | 3.84" | +6.6 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 88.6" | 90.1" | +1.7 % |
| Revs per mile | 715 | 703 | -1.7 % |
| Axle height | 14.10" | 14.34" | +6.0 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 305/30R21. Fit 325/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,168 on the road, so it under-counts by 168. At a true 70 mph the first turns 834 times a minute, the second 820.