315/30R23 vs 325/30R23
Going from one to the other changes the overall height by +0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/30R23 | 325/30R23 | Change |
|---|---|---|---|
| Overall height | 30.4" | 30.7" | +0.8 % |
| Sidewall | 3.72" | 3.84" | +3.2 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 95.6" | 96.4" | +0.8 % |
| Revs per mile | 663 | 657 | -0.8 % |
| Axle height | 15.22" | 15.34" | +3.0 mm |
| Approved rim width | 8.5–11.5" | 9–12" | overlap 9–11.5" |
Both are approved on a rim 9" to 11.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 315/30R23. Fit 325/30R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.5 mph | +0.5 |
| 90 mph | 90.7 mph | +0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,078 on the road, so it under-counts by 78. At a true 70 mph the first turns 773 times a minute, the second 767.