325/50R20 vs 305/55R20
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 | 325/50R20 | 305/55R20 | Change |
|---|---|---|---|
| Overall height | 32.8" | 33.2" | +1.3 % |
| Sidewall | 6.40" | 6.60" | +3.2 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 103.0" | 104.3" | +1.3 % |
| Revs per mile | 615 | 607 | -1.3 % |
| Axle height | 16.40" | 16.60" | +5.3 mm |
| Approved rim width | 9–12" | 8.5–11" | 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 325/50R20. Fit 305/55R20 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 | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,126 on the road, so it under-counts by 126. At a true 70 mph the first turns 717 times a minute, the second 709.