Abstract: The Umbrella Semi-Trailer Drag Reducer is a device that reduces tractor-trailer aerodynamic drag, saving energy and reducing fuel consumption. The design includes a number of features similar to an umbrella, including flexible skin, metal ribs, quick-acting deployment, and compact stowage. The Umbrella Semi-Trailer Drag Reducer is designed to minimize installed cost, be robust and vandal resistant, and reduce the time needed for deployment to only a few seconds. The design includes two independent, paraboloid, mirrored halves, one mounted to each door. The bottom of each mirrored half is open, with a profile similar to one quadrant of an umbrella. Wide-throw hinges accommodate the collapsed stowed drag reducer. Turnbuckles maintain taught skin. The design allows easy retrofit for most existing semi-trailers. The drag reducer is mounted high above the road surface, with no part protruding beyond the top or sides of the semi-trailer.