305/55R20 vs 285/60R20
Going from one to the other changes the overall height by +0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/55R20 | 285/60R20 | Change |
|---|---|---|---|
| Overall height | 33.2" | 33.5" | +0.8 % |
| Sidewall | 6.60" | 6.73" | +1.9 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 104.3" | 105.1" | +0.8 % |
| Revs per mile | 607 | 603 | -0.8 % |
| Axle height | 16.60" | 16.73" | +3.3 mm |
| Approved rim width | 8.5–11" | 8–10.5" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" 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 285/60R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.5 mph | +0.5 |
| 90 mph | 90.7 mph | +0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,077 on the road, so it under-counts by 77. At a true 70 mph the first turns 709 times a minute, the second 703.