315/45R17 vs 325/45R17
Going from one to the other changes the overall height by +1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/45R17 | 325/45R17 | Change |
|---|---|---|---|
| Overall height | 28.2" | 28.5" | +1.3 % |
| Sidewall | 5.58" | 5.76" | +3.2 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 88.5" | 89.6" | +1.3 % |
| Revs per mile | 716 | 707 | -1.3 % |
| Axle height | 14.08" | 14.26" | +4.5 mm |
| Approved rim width | 8.5–11.5" | 9–12" | 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 315/45R17. Fit 325/45R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.9 mph | +0.9 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,126 on the road, so it under-counts by 126. At a true 70 mph the first turns 836 times a minute, the second 825.