An aerodynamic drag reducer in the form of an air deflector fairing is provided for a trailer which is secured to the lower ends of each of the lower side rails of the trailer. Each of the fairings have upper outer ends which are secured to the respective lower side rail and which extends downwardly and inwardly therefrom, then upwardly and inwardly therefrom and then inwardly therefrom. The inner ends of the fairings are secured to the cross-members of the trailer. The drag reducers of this invention reduce the turbulence experienced by cross-winds subjected to the trailer thereby reducing the aerodynamic drag of the cross-winds.