305/40R21 vs 325/40R21
Going from one to the other changes the overall height by +2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/40R21 | 325/40R21 | Change |
|---|---|---|---|
| Overall height | 30.6" | 31.2" | +2.1 % |
| Sidewall | 4.80" | 5.12" | +6.6 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 96.2" | 98.1" | +2.1 % |
| Revs per mile | 659 | 646 | -2.1 % |
| Axle height | 15.30" | 15.62" | +8.0 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 305/40R21. Fit 325/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.9 mph | +1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,206 on the road, so it under-counts by 206. At a true 70 mph the first turns 769 times a minute, the second 753.