325/35R23 vs 305/40R23
Going from one to the other changes the overall height by +2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/35R23 | 305/40R23 | Change |
|---|---|---|---|
| Overall height | 32.0" | 32.6" | +2.0 % |
| Sidewall | 4.48" | 4.80" | +7.3 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 100.4" | 102.4" | +2.0 % |
| Revs per mile | 631 | 619 | -2.0 % |
| Axle height | 15.98" | 16.30" | +8.3 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 325/35R23. Fit 305/40R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,203 on the road, so it under-counts by 203. At a true 70 mph the first turns 736 times a minute, the second 722.