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 according to our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a bottom view thereof;
FIG. 7 is a fragmentary enlarged front view showing the details of the tread portion thereof; and,
FIG. 8 is an end view taken along line 8-8 in FIG. 7 thereof.
Broken lines in the drawing figures are intended to show portions of the tire that are not being claimed, and thus form no part of the claimed invention.