Abstract: In combination for trailers and bob tail trucks, an air spoiler and an air foil mounted in assembly together. The air spoiler having a generally conical shape and being hollow at its annular open end. Means mounting structure is secured with the air spoiler adjacent to its annular open end for attaching the air spoiler to a truck trailer and the like. The generally conical shape includes a gently curving upper surface defined by an outer generally arcuately curved continuous lead edge and an under curved surface continuously joining the continuous curved lead edge with the open end of the air spoiler. The air foil has mounting means supporting it on the gently curving upper surface generally at its rear most upper end. The air foil has a generally arched shape which shape is arched in a direction away from the gently curving upper surface.