345/35R19 vs 325/40R19
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 | 345/35R19 | 325/40R19 | Change |
|---|---|---|---|
| Overall height | 28.5" | 29.2" | +2.6 % |
| Sidewall | 4.75" | 5.12" | +7.7 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 89.6" | 91.8" | +2.6 % |
| Revs per mile | 707 | 690 | -2.6 % |
| Axle height | 14.25" | 14.62" | +9.3 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 345/35R19. Fit 325/40R19 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,255 on the road, so it under-counts by 255. At a true 70 mph the first turns 825 times a minute, the second 805.