345/40R22 vs 325/40R22
Going from one to the other changes the overall height by -1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/40R22 | 325/40R22 | Change |
|---|---|---|---|
| Overall height | 32.9" | 32.2" | -1.9 % |
| Sidewall | 5.43" | 5.12" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 103.3" | 101.3" | -1.9 % |
| Revs per mile | 614 | 626 | +1.9 % |
| Axle height | 16.43" | 16.12" | -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 345/40R22. Fit 325/40R22 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 | 68.7 mph | -1.3 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,808 on the road, so it over-counts by 192. At a true 70 mph the first turns 716 times a minute, the second 730.