315/25R23 vs 325/25R23
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 | 315/25R23 | 325/25R23 | Change |
|---|---|---|---|
| Overall height | 29.2" | 29.4" | +0.7 % |
| Sidewall | 3.10" | 3.20" | +3.2 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 91.7" | 92.4" | +0.7 % |
| Revs per mile | 691 | 686 | -0.7 % |
| Axle height | 14.60" | 14.70" | +2.5 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 315/25R23. Fit 325/25R23 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 806 times a minute, the second 800.