305/30R17 vs 325/30R17
Going from one to the other changes the overall height by +2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/30R17 | 325/30R17 | Change |
|---|---|---|---|
| Overall height | 24.2" | 24.7" | +2.0 % |
| Sidewall | 3.60" | 3.84" | +6.6 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 76.0" | 77.5" | +2.0 % |
| Revs per mile | 833 | 817 | -2.0 % |
| Axle height | 12.10" | 12.34" | +6.0 mm |
| Approved rim width | 8.5–11" | 9–12" | 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 305/30R17. Fit 325/30R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,195 on the road, so it under-counts by 195. At a true 70 mph the first turns 972 times a minute, the second 953.