305/50R20 vs 325/45R20
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 | 305/50R20 | 325/45R20 | Change |
|---|---|---|---|
| Overall height | 32.0" | 31.5" | -1.5 % |
| Sidewall | 6.00" | 5.76" | -4.1 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 100.6" | 99.0" | -1.5 % |
| Revs per mile | 630 | 640 | +1.5 % |
| Axle height | 16.00" | 15.76" | -6.3 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 305/50R20. Fit 325/45R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.9 mph | -1.1 |
| 90 mph | 88.6 mph | -1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,846 on the road, so it over-counts by 154. At a true 70 mph the first turns 735 times a minute, the second 747.
What drivers reported
305/50R20 carries 4 complaints. Nobody has filed one naming 325/45R20.