Abstract: Airstream deflector assembly (D) on the underside (16) of a large vehicle such as a semi-trailer (T) or the like, to reduce drag, improve fuel economy, and reduce turbulence of rain or snow on a roadway (R) traveled by the vehicle, such assembly comprising a main panel (30) which is essentially flat and situated well forward of the following tires (14) of the vehicle and deflecting the airstream downwardly primarily below the following axle (15) and centers of such following tires (14), such main deflection panel (D) having mounted thereon four lateral airstream deflectors, two (80) outboardly and two (90) inboardly of the panel (30) which serve to concentrate the deflected airstream laterally toward the following tires (14) of the vehicle and away from the "tunnel" between the tires (14).