345/50R21 vs 325/50R21
Going from one to the other changes the overall height by -2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/50R21 | 325/50R21 | Change |
|---|---|---|---|
| Overall height | 34.6" | 33.8" | -2.3 % |
| Sidewall | 6.79" | 6.40" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 108.6" | 106.2" | -2.3 % |
| Revs per mile | 583 | 597 | +2.3 % |
| Axle height | 17.29" | 16.90" | -10.0 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 345/50R21. Fit 325/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,772 on the road, so it over-counts by 228. At a true 70 mph the first turns 680 times a minute, the second 696.