325/30R24 vs 345/25R24
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/30R24 | 345/25R24 | Change |
|---|---|---|---|
| Overall height | 31.7" | 30.8" | -2.8 % |
| Sidewall | 3.84" | 3.40" | -11.5 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 99.5" | 96.7" | -2.8 % |
| Revs per mile | 637 | 655 | +2.8 % |
| Axle height | 15.84" | 15.40" | -11.3 mm |
| Approved rim width | 9–12" | 9.5–12.5" | 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 325/30R24. Fit 345/25R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,720 on the road, so it over-counts by 280. At a true 70 mph the first turns 743 times a minute, the second 764.