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, wherein top, bottom and rear views are identical and thus omitted;
FIG. 3 is a left side view thereof, wherein the right side view is identical and thus omitted;
FIG. 4 is a fragmentary enlarged view showing the details of the tread portion thereof; and,
FIG. 5 is an end view taken along line A-A in FIG. 4.
In the drawings, the broken lines defining the sidewall and the inner bead of the tire depict environmental subject matter that forms no part of the claimed design.