345/40R21 vs 325/40R21
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/40R21 | 325/40R21 | Change |
|---|---|---|---|
| Overall height | 31.9" | 31.2" | -2.0 % |
| Sidewall | 5.43" | 5.12" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 100.1" | 98.1" | -2.0 % |
| Revs per mile | 633 | 646 | +2.0 % |
| Axle height | 15.93" | 15.62" | -8.0 mm |
| Approved rim width | 9.5–12.5" | 9–12" | overlap 9.5–12" |
Both are approved on a rim 9.5" to 12" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 345/40R21. Fit 325/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,802 on the road, so it over-counts by 198. At a true 70 mph the first turns 738 times a minute, the second 753.