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 rear view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right 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.
In the drawings, the broken lines defining the sidewall, the unadorned portion of the tread and the inner bead of the tire depict environmental subject matter that forms no part of the claimed design.