315/45R20 vs 325/40R20
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/45R20 | 325/40R20 | Change |
|---|---|---|---|
| Overall height | 31.2" | 30.2" | -3.0 % |
| Sidewall | 5.58" | 5.12" | -8.3 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 97.9" | 95.0" | -3.0 % |
| Revs per mile | 647 | 667 | +3.0 % |
| Axle height | 15.58" | 15.12" | -11.8 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 315/45R20. Fit 325/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,703 on the road, so it over-counts by 297. At a true 70 mph the first turns 755 times a minute, the second 778.