305/30R23 vs 325/25R23
Going from one to the other changes the overall height by -2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/30R23 | 325/25R23 | Change |
|---|---|---|---|
| Overall height | 30.2" | 29.4" | -2.7 % |
| Sidewall | 3.60" | 3.20" | -11.2 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 94.9" | 92.4" | -2.7 % |
| Revs per mile | 668 | 686 | +2.7 % |
| Axle height | 15.10" | 14.70" | -10.3 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 305/30R23. Fit 325/25R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,733 on the road, so it over-counts by 267. At a true 70 mph the first turns 779 times a minute, the second 800.