305/55R17 vs 325/50R17
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/55R17 | 325/50R17 | Change |
|---|---|---|---|
| Overall height | 30.2" | 29.8" | -1.4 % |
| Sidewall | 6.60" | 6.40" | -3.1 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 94.9" | 93.6" | -1.4 % |
| Revs per mile | 668 | 677 | +1.4 % |
| Axle height | 15.10" | 14.90" | -5.3 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 305/55R17. Fit 325/50R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,863 on the road, so it over-counts by 137. At a true 70 mph the first turns 779 times a minute, the second 790.