305/55R20 vs 325/55R20
Going from one to the other changes the overall height by +2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/55R20 | 325/55R20 | Change |
|---|---|---|---|
| Overall height | 33.2" | 34.1" | +2.6 % |
| Sidewall | 6.60" | 7.04" | +6.6 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 104.3" | 107.0" | +2.6 % |
| Revs per mile | 607 | 592 | -2.6 % |
| Axle height | 16.60" | 17.04" | +11.0 mm |
| Approved rim width | 8.5–11" | 9–12" | overlap 9–11" |
Both are approved on a rim 9" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/55R20. Fit 325/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,261 on the road, so it under-counts by 261. At a true 70 mph the first turns 709 times a minute, the second 691.
What drivers reported
305/55R20 carries 1 complaints. Nobody has filed one naming 325/55R20.