305/60R20 vs 285/65R20
Going from one to the other changes the overall height by +0.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/60R20 | 285/65R20 | Change |
|---|---|---|---|
| Overall height | 34.4" | 34.6" | +0.5 % |
| Sidewall | 7.20" | 7.29" | +1.2 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 108.1" | 108.7" | +0.5 % |
| Revs per mile | 586 | 583 | -0.5 % |
| Axle height | 17.20" | 17.29" | +2.3 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 305/60R20. Fit 285/65R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.4 mph | +0.4 |
| 90 mph | 90.5 mph | +0.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,051 on the road, so it under-counts by 51. At a true 70 mph the first turns 684 times a minute, the second 680.
What drivers reported
Nobody has filed a complaint naming 305/60R20. 285/65R20 carries 2.