A variable geometry aerodynamic fairing is described for reduction of the base drag of a tractor trailer traveling at highway speed. The base drag reduction is provided by a set of two adjustable rectangular fairings which can be opened from a flat configuration when folded one against each side door, into a set of curved airfoil shaped fairings extending into the boundary layer adjacent to the trailer sides and cambered such as to turn a portion of the mainstream side air flow into the base wake region of the trailer. The cambered fairings thus reduce the strength of the standing base wake vortices and hence, the resultant negative base pressures (drag) formed in the trailer wake region. A direct application of this device would be to improve the fuel economy of a large portion of the U.S. trucking fleet during highway transit, while still providing rapid fairing retraction into a thin, flat stowed position against the trailer doors for unloading at destination.