315/25R20 vs 325/25R20
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 | 315/25R20 | 325/25R20 | Change |
|---|---|---|---|
| Overall height | 26.2" | 26.4" | +0.8 % |
| Sidewall | 3.10" | 3.20" | +3.2 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 82.3" | 82.9" | +0.8 % |
| Revs per mile | 770 | 764 | -0.8 % |
| Axle height | 13.10" | 13.20" | +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 315/25R20. Fit 325/25R20 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 10,075 on the road, so it under-counts by 75. At a true 70 mph the first turns 898 times a minute, the second 891.