The ornamental design for a tread portion of an automobile tire, as shown and described.
FIG. 1 is a perspective view of a tread portion of an automobile tire;
FIG. 2 is a front view thereof;
FIG. 3 is a plane view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a fragmentary enlarged view showing the details of the tread portion thereof;
FIG. 6 is an end view taken along line 6-6 in FIG. 5; and,
FIG. 7 is an end view taken along line 7-7 in FIG. 5.
Broken lines in the drawing figures are used to show environmental structure as well as a boundary between the claimed and unclaimed subject matter, and thus form no part of the claimed invention.