345/25R21 vs 325/25R21
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/25R21 | 325/25R21 | Change |
|---|---|---|---|
| Overall height | 27.8" | 27.4" | -1.4 % |
| Sidewall | 3.40" | 3.20" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 87.3" | 86.1" | -1.4 % |
| Revs per mile | 726 | 736 | +1.4 % |
| Axle height | 13.90" | 13.70" | -5.0 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 345/25R21. Fit 325/25R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,858 on the road, so it over-counts by 142. At a true 70 mph the first turns 847 times a minute, the second 859.