305/45R21 vs 325/40R21
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/45R21 | 325/40R21 | Change |
|---|---|---|---|
| Overall height | 31.8" | 31.2" | -1.8 % |
| Sidewall | 5.40" | 5.12" | -5.3 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 99.9" | 98.1" | -1.8 % |
| Revs per mile | 634 | 646 | +1.8 % |
| Axle height | 15.90" | 15.62" | -7.3 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 305/45R21. Fit 325/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.4 mph | -1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,821 on the road, so it over-counts by 179. At a true 70 mph the first turns 740 times a minute, the second 753.