325/30R23 vs 305/35R23
Going from one to the other changes the overall height by +2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/30R23 | 305/35R23 | Change |
|---|---|---|---|
| Overall height | 30.7" | 31.4" | +2.4 % |
| Sidewall | 3.84" | 4.20" | +9.5 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 96.4" | 98.7" | +2.4 % |
| Revs per mile | 657 | 642 | -2.4 % |
| Axle height | 15.34" | 15.70" | +9.3 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 325/30R23. Fit 305/35R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.1 mph | +2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,237 on the road, so it under-counts by 237. At a true 70 mph the first turns 767 times a minute, the second 749.