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.
Broken lines in the drawing figures are intended to show environmental structure only, and form no part of the claimed design.
The broken lines showing the area between the tread portions is for the purpose of illustrating portions of the tire and form no part of the claimed design.