325/40R24 vs 315/45R24
Going from one to the other changes the overall height by +2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/40R24 | 315/45R24 | Change |
|---|---|---|---|
| Overall height | 34.2" | 35.2" | +2.7 % |
| Sidewall | 5.12" | 5.58" | +9.0 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 107.6" | 110.5" | +2.7 % |
| Revs per mile | 589 | 574 | -2.7 % |
| Axle height | 17.12" | 17.58" | +11.8 mm |
| Approved rim width | 9–12" | 8.5–11.5" | 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 325/40R24. Fit 315/45R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,270 on the road, so it under-counts by 270. At a true 70 mph the first turns 687 times a minute, the second 669.