Abstract: A rooftop drag reducing device includes a lower base portion fixedly mounted to the roof of a truck for semi-trailers. An upper portion of the drag reducing device is selectively movable relative to the base portion between a fully extended operating position, and a fully retracted storage position. In the fully retracted position, the upper portion is received within the lower portion such that at least part of the top surface of the lower portion is above the top surface of the upper portion. The design in accordance with the present invention increases the overall stability of the drag reducing device in both the extended and retracted positions, provides enhanced protection for the upper portion in its retracted position fully received within the lower portion, provides greater clearance than is possible with known drag reduction devices, and increases the maximum elevation within the cab or sleeper compartment of the vehicle to which the drag reduction device is mounted.