315/40R24 vs 325/40R24
Going from one to the other changes the overall height by +0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/40R24 | 325/40R24 | Change |
|---|---|---|---|
| Overall height | 33.9" | 34.2" | +0.9 % |
| Sidewall | 4.96" | 5.12" | +3.2 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 106.6" | 107.6" | +0.9 % |
| Revs per mile | 595 | 589 | -0.9 % |
| Axle height | 16.96" | 17.12" | +4.0 mm |
| Approved rim width | 8.5–11.5" | 9–12" | overlap 9–11.5" |
Both are approved on a rim 9" to 11.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 315/40R24. Fit 325/40R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.8 mph | +0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,093 on the road, so it under-counts by 93. At a true 70 mph the first turns 694 times a minute, the second 687.