305/25R20 vs 325/25R20
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/25R20 | 325/25R20 | Change |
|---|---|---|---|
| Overall height | 26.0" | 26.4" | +1.5 % |
| Sidewall | 3.00" | 3.20" | +6.6 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 81.7" | 82.9" | +1.5 % |
| Revs per mile | 776 | 764 | -1.5 % |
| Axle height | 13.00" | 13.20" | +5.0 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 305/25R20. Fit 325/25R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.4 mph | +1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,151 on the road, so it under-counts by 151. At a true 70 mph the first turns 905 times a minute, the second 891.