325/25R17 vs 305/25R17
Going from one to the other changes the overall height by -1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/25R17 | 305/25R17 | Change |
|---|---|---|---|
| Overall height | 23.4" | 23.0" | -1.7 % |
| Sidewall | 3.20" | 3.00" | -6.2 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 73.5" | 72.3" | -1.7 % |
| Revs per mile | 862 | 877 | +1.7 % |
| Axle height | 11.70" | 11.50" | -5.0 mm |
| Approved rim width | 9–12" | 8.5–11" | overlap 9–11" |
Both are approved on a rim 9" to 11" 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 305/25R17 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 | 68.8 mph | -1.2 |
| 90 mph | 88.5 mph | -1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,832 on the road, so it over-counts by 168. At a true 70 mph the first turns 1006 times a minute, the second 1023.