325/40R23 vs 305/40R23
Going from one to the other changes the overall height by -1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/40R23 | 305/40R23 | Change |
|---|---|---|---|
| Overall height | 33.2" | 32.6" | -1.9 % |
| Sidewall | 5.12" | 4.80" | -6.2 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 104.4" | 102.4" | -1.9 % |
| Revs per mile | 607 | 619 | +1.9 % |
| Axle height | 16.62" | 16.30" | -8.0 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 325/40R23. Fit 305/40R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,810 on the road, so it over-counts by 190. At a true 70 mph the first turns 708 times a minute, the second 722.