325/30R21 vs 305/30R21
Going from one to the other changes the overall height by -1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/30R21 | 305/30R21 | Change |
|---|---|---|---|
| Overall height | 28.7" | 28.2" | -1.6 % |
| Sidewall | 3.84" | 3.60" | -6.2 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 90.1" | 88.6" | -1.6 % |
| Revs per mile | 703 | 715 | +1.6 % |
| Axle height | 14.34" | 14.10" | -6.0 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 325/30R21. Fit 305/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.5 mph | -1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,835 on the road, so it over-counts by 165. At a true 70 mph the first turns 820 times a minute, the second 834.