345/30R22 vs 325/30R22
Going from one to the other changes the overall height by -1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/30R22 | 325/30R22 | Change |
|---|---|---|---|
| Overall height | 30.1" | 29.7" | -1.6 % |
| Sidewall | 4.07" | 3.84" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 94.7" | 93.2" | -1.6 % |
| Revs per mile | 669 | 680 | +1.6 % |
| Axle height | 15.07" | 14.84" | -6.0 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 345/30R22. Fit 325/30R22 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.9 mph | -1.1 |
| 90 mph | 88.6 mph | -1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,843 on the road, so it over-counts by 157. At a true 70 mph the first turns 780 times a minute, the second 793.