315/55R20 vs 325/55R20
Going from one to the other changes the overall height by +1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/55R20 | 325/55R20 | Change |
|---|---|---|---|
| Overall height | 33.6" | 34.1" | +1.3 % |
| Sidewall | 6.82" | 7.04" | +3.2 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 105.7" | 107.0" | +1.3 % |
| Revs per mile | 599 | 592 | -1.3 % |
| Axle height | 16.82" | 17.04" | +5.5 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 315/55R20. Fit 325/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.9 mph | +0.9 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,129 on the road, so it under-counts by 129. At a true 70 mph the first turns 699 times a minute, the second 691.