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 fragmentary enlarged front view showing the details of the tread portion thereof; and,
FIG. 6 is a section view taken along line 6-6 in FIG. 5.
The dash-dash broken lines are intended to show portions of the tire that are not being claimed and thus form no part of the claimed design. The dash-dot-dash broken lines represent boundaries of the claim.