A semitrailer underbody screen and wheel-set fairing kit comprises several panels and fairings for retrofitting on conventional semitrailers. A sectioned panel like fish scales covers the dozens of transverse I-beam cross-members positioned underneath conventional enclosed trailer floors, and carries on down the length of the underbody. A v-hull air deflector is set just beneath these panel covers, ahead of the semitrailer's undercarriage, suspension and tandem rear axle wheels. Such v-hull air deflector splits and aerodynamically “flows” the air being pushed ahead outwards to the sides and around the wheels. The tandem rear axle wheels themselves are completely boxed inside full fenders that cowl over the front, sides, top, and rear. Removable access panels are provided on the outsides of the full fender units for wheel and tire maintenance. The result is significantly less wind drag and noise at highway cruising speeds. Less drag means better fuel economy for the tractor operator.