325/65R17 vs 315/65R17
Going from one to the other changes the overall height by -1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/65R17 | 315/65R17 | Change |
|---|---|---|---|
| Overall height | 33.6" | 33.1" | -1.5 % |
| Sidewall | 8.32" | 8.06" | -3.1 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 105.7" | 104.1" | -1.5 % |
| Revs per mile | 600 | 609 | +1.5 % |
| Axle height | 16.82" | 16.56" | -6.5 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 325/65R17. Fit 315/65R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.9 mph | -1.1 |
| 90 mph | 88.6 mph | -1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,848 on the road, so it over-counts by 152. At a true 70 mph the first turns 700 times a minute, the second 710.