325/55R20 vs 305/60R20
Going from one to the other changes the overall height by +1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/55R20 | 305/60R20 | Change |
|---|---|---|---|
| Overall height | 34.1" | 34.4" | +1.0 % |
| Sidewall | 7.04" | 7.20" | +2.4 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 107.0" | 108.1" | +1.0 % |
| Revs per mile | 592 | 586 | -1.0 % |
| Axle height | 17.04" | 17.20" | +4.3 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 325/55R20. Fit 305/60R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 90.9 mph | +0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,098 on the road, so it under-counts by 98. At a true 70 mph the first turns 691 times a minute, the second 684.