325/60R23 vs 305/65R23
Going from one to the other changes the overall height by +0.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/60R23 | 305/65R23 | Change |
|---|---|---|---|
| Overall height | 38.4" | 38.6" | +0.7 % |
| Sidewall | 7.68" | 7.81" | +1.7 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 120.5" | 121.3" | +0.7 % |
| Revs per mile | 526 | 522 | -0.7 % |
| Axle height | 19.18" | 19.31" | +3.3 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 325/60R23. Fit 305/65R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.5 mph | +0.5 |
| 90 mph | 90.6 mph | +0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,067 on the road, so it under-counts by 67. At a true 70 mph the first turns 613 times a minute, the second 609.