325/25R17 vs 315/25R17
Going from one to the other changes the overall height by -0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/25R17 | 315/25R17 | Change |
|---|---|---|---|
| Overall height | 23.4" | 23.2" | -0.8 % |
| Sidewall | 3.20" | 3.10" | -3.1 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 73.5" | 72.9" | -0.8 % |
| Revs per mile | 862 | 869 | +0.8 % |
| Axle height | 11.70" | 11.60" | -2.5 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 325/25R17. Fit 315/25R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.2 mph | -0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,916 on the road, so it over-counts by 84. At a true 70 mph the first turns 1006 times a minute, the second 1014.