I claim the ornamental design for a weight for truck mud flaps, as shown and described.
FIG. 1 is a front and left side perspective view of a weight for truck mud flaps showing my new design;
FIG. 2 is a front and right side perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a left side elevation view thereof; and,
FIG. 5 is a top plan view thereof.
The broken lines in the drawings illustrate portions of the weight for truck mud flaps and an environmental truck that do not form part of the claimed design.