Trailer tail

A trailer tail, boat tail, or rear fairing is an aerodynamic device intended to improve the fuel economy of semi-trailer trucks. It comprises a set of panels, usually collapsible, which fold out from the rear of the trailer, creating a tapered shape that reduces drag from the low-pressure wake created behind the trailer. Trailer tails are one form of aerodynamic technology verified by the U.S. Environmental Protection Agency's SmartWay Transport Partnership. Trailer tails alone have demonstrated a fuel savings of 1%–5%, and in concert with trailer skirts, 9% improvement has been demonstrated.
TrailerTail is a registered trademark of STEMCO.
SuperTruck project.
At least one model of trailer tail, trade named TrailerTail, is part of Navistar's SuperTruck project, which will use $37 million of US Department of Energy grants to build next generation trucks and tractor trailers.