325/45R23 vs 305/45R23
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/45R23 | 305/45R23 | Change |
|---|---|---|---|
| Overall height | 34.5" | 33.8" | -2.1 % |
| Sidewall | 5.76" | 5.40" | -6.2 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 108.4" | 106.2" | -2.1 % |
| Revs per mile | 584 | 597 | +2.1 % |
| Axle height | 17.26" | 16.90" | -9.0 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 325/45R23. Fit 305/45R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,795 on the road, so it over-counts by 205. At a true 70 mph the first turns 682 times a minute, the second 696.