325/25R23 vs 305/25R23
Going from one to the other changes the overall height by -1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/25R23 | 305/25R23 | Change |
|---|---|---|---|
| Overall height | 29.4" | 29.0" | -1.3 % |
| Sidewall | 3.20" | 3.00" | -6.2 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 92.4" | 91.1" | -1.3 % |
| Revs per mile | 686 | 695 | +1.3 % |
| Axle height | 14.70" | 14.50" | -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 325/25R23. Fit 305/25R23 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 | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,866 on the road, so it over-counts by 134. At a true 70 mph the first turns 800 times a minute, the second 811.