345/40R19 vs 325/40R19
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/40R19 | 325/40R19 | Change |
|---|---|---|---|
| Overall height | 29.9" | 29.2" | -2.1 % |
| Sidewall | 5.43" | 5.12" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 93.8" | 91.8" | -2.1 % |
| Revs per mile | 675 | 690 | +2.1 % |
| Axle height | 14.93" | 14.62" | -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 345/40R19. Fit 325/40R19 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,789 on the road, so it over-counts by 211. At a true 70 mph the first turns 788 times a minute, the second 805.