The ornamental design for a tread portion of an automobile tire, as shown and described.
FIG. 1 is a front view of a tread portion of an automobile tire;
FIG. 2 is a perspective view thereof;
FIG. 3 is a left side view thereof; and,
FIG. 4 is a fragmentary enlarged view showing the details of the tread portion thereof.
The evenly-spaced broken lines shown in the drawings are for the purpose of showing environmental structure only and form no part of the claimed design. The dashed-dotted broken lines in the drawings are used to show the boundary between claimed and unclaimed subject matter and form no part of the claimed design.