305/25R24 vs 315/25R24
Going from one to the other changes the overall height by +0.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/25R24 | 315/25R24 | Change |
|---|---|---|---|
| Overall height | 30.0" | 30.2" | +0.7 % |
| Sidewall | 3.00" | 3.10" | +3.3 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 94.3" | 94.9" | +0.7 % |
| Revs per mile | 672 | 668 | -0.7 % |
| Axle height | 15.00" | 15.10" | +2.5 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 305/25R24. Fit 315/25R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.5 mph | +0.5 |
| 90 mph | 90.6 mph | +0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,066 on the road, so it under-counts by 66. At a true 70 mph the first turns 784 times a minute, the second 779.