345/30R24 vs 325/30R24
Going from one to the other changes the overall height by -1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/30R24 | 325/30R24 | Change |
|---|---|---|---|
| Overall height | 32.1" | 31.7" | -1.5 % |
| Sidewall | 4.07" | 3.84" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 101.0" | 99.5" | -1.5 % |
| Revs per mile | 627 | 637 | +1.5 % |
| Axle height | 16.07" | 15.84" | -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 345/30R24. Fit 325/30R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,853 on the road, so it over-counts by 147. At a true 70 mph the first turns 732 times a minute, the second 743.