325/25R20 vs 315/25R20
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 | 325/25R20 | 315/25R20 | Change |
|---|---|---|---|
| Overall height | 26.4" | 26.2" | -0.7 % |
| Sidewall | 3.20" | 3.10" | -3.1 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 82.9" | 82.3" | -0.7 % |
| Revs per mile | 764 | 770 | +0.7 % |
| Axle height | 13.20" | 13.10" | -2.5 mm |
| Approved rim width | 9–12" | 8.5–11.5" | 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 325/25R20. Fit 315/25R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.5 mph | -0.5 |
| 90 mph | 89.3 mph | -0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,925 on the road, so it over-counts by 75. At a true 70 mph the first turns 891 times a minute, the second 898.