285/60R20 is 0.61 in shorter than 275/65R20 (−1.79%) and 0.39 in wider; at an indicated 60 mph you are doing 58.9 mph.
Both overall diameters at one scale, sharing a center. The detail beside them enlarges the one place they differ, and its dimension is the difference the table below states.
The difference, one row at a time
275/65R20 and 285/60R20, one measurement per row; the difference is 285/60R20 minus 275/65R20