A drag reducing apparatus is hingedly mounted to the rear of a truck or trailer vehicle so that the apparatus can be pivoted clear of the rear doors of the vehicle. The apparatus includes a hinge assembly that allows the apparatus to open substantially flat against the side of the vehicle so as not to impede the full movement of the doors. The apparatus includes an arrangement of connected panels that are initially folded flat against the back of the vehicle but automatically open to a drag reducing configuration at a sufficient vehicle speed. The connected panels are further configured so that the force of gravity automatically returns the panels to their initial folded configuration when the vehicle speed falls below a threshold. The apparatus incorporates a modular construction that allows replacement of individual panel sections. The mounting frame is constructed to augment the drag reduction characteristics of the apparatus.