305/45R24 vs 315/40R24
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/45R24 | 315/40R24 | Change |
|---|---|---|---|
| Overall height | 34.8" | 33.9" | -2.5 % |
| Sidewall | 5.40" | 4.96" | -8.2 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 109.3" | 106.6" | -2.5 % |
| Revs per mile | 579 | 595 | +2.5 % |
| Axle height | 17.40" | 16.96" | -11.3 mm |
| Approved rim width | 8.5–11" | 8.5–11.5" | overlap 8.5–11" |
Both are approved on a rim 8.5" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/45R24. Fit 315/40R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,746 on the road, so it over-counts by 254. At a true 70 mph the first turns 676 times a minute, the second 694.