The ornamental design for a fueltankanti-theft device, as shown and described.
FIG. 1 is a perspective view of a fuel tank anti-theft device showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front elevational view thereof with the rear elevational view, the right side elevational view, and the left side elevational view being identical to the front elevational view; and,
FIG. 5 is an alternate elevational view the fuel tank anti-theft device being rotated 45° from the orientation shown in FIG. 4.