325/40R20 vs 315/40R20
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/40R20 | 315/40R20 | Change |
|---|---|---|---|
| Overall height | 30.2" | 29.9" | -1.0 % |
| Sidewall | 5.12" | 4.96" | -3.1 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 95.0" | 94.0" | -1.0 % |
| Revs per mile | 667 | 674 | +1.0 % |
| Axle height | 15.12" | 14.96" | -4.0 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 325/40R20. Fit 315/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,896 on the road, so it over-counts by 104. At a true 70 mph the first turns 778 times a minute, the second 786.