345/65R21 vs 325/65R21
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/65R21 | 325/65R21 | Change |
|---|---|---|---|
| Overall height | 38.7" | 37.6" | -2.6 % |
| Sidewall | 8.83" | 8.32" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 121.4" | 118.2" | -2.6 % |
| Revs per mile | 522 | 536 | +2.6 % |
| Axle height | 19.33" | 18.82" | -13.0 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 345/65R21. Fit 325/65R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,735 on the road, so it over-counts by 265. At a true 70 mph the first turns 609 times a minute, the second 625.