345/35R24 vs 325/40R24
Going from one to the other changes the overall height by +2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/35R24 | 325/40R24 | Change |
|---|---|---|---|
| Overall height | 33.5" | 34.2" | +2.2 % |
| Sidewall | 4.75" | 5.12" | +7.7 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 105.3" | 107.6" | +2.2 % |
| Revs per mile | 602 | 589 | -2.2 % |
| Axle height | 16.75" | 17.12" | +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 345/35R24. Fit 325/40R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.5 mph | +1.5 |
| 90 mph | 92.0 mph | +2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,217 on the road, so it under-counts by 217. At a true 70 mph the first turns 702 times a minute, the second 687.