The problem of evaluating the fuel savings effectiveness of a truck aerodynamic drag reducing device in an over-the-road test is considered. The sensitivity of a vehicle's fuel consumption rate to factors other than the drag reduction produced by a device is discussed to illustrate potential sources of error. Evaluations of typical performances of several different drag reducing designs demonstrate how effectiveness varies as a function of the design of a device and the short-term and the long-term wind conditions under which it operates. Current SAE over-the-road test procedures are discussed in terms of the correlation between the test result and the probable long-term final savings produced by a device. Results from several fleet experiments are reviewed to illustrate differences in fuel savings results as functions of the drag reduction potential of the vehicle, the design of the device, the test conditions, the test technique employed, and the method of data interpretation. Guidelines for the meaningful evaluation of the effectiveness of a drag reducing device are presented.