325/45R17 vs 305/50R17
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 | 325/45R17 | 305/50R17 | Change |
|---|---|---|---|
| Overall height | 28.5" | 29.0" | +1.7 % |
| Sidewall | 5.76" | 6.00" | +4.3 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 89.6" | 91.1" | +1.7 % |
| Revs per mile | 707 | 695 | -1.7 % |
| Axle height | 14.26" | 14.50" | +6.3 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 325/45R17. Fit 305/50R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.6 mph | +1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,173 on the road, so it under-counts by 173. At a true 70 mph the first turns 825 times a minute, the second 811.