325/30R17 vs 315/30R17
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/30R17 | 315/30R17 | Change |
|---|---|---|---|
| Overall height | 24.7" | 24.4" | -1.0 % |
| Sidewall | 3.84" | 3.72" | -3.1 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 77.5" | 76.8" | -1.0 % |
| Revs per mile | 817 | 825 | +1.0 % |
| Axle height | 12.34" | 12.22" | -3.0 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 325/30R17. Fit 315/30R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,904 on the road, so it over-counts by 96. At a true 70 mph the first turns 953 times a minute, the second 963.