Abstract: An air foil system for detachable adjustable mounting to the frame of a truck tractor rearwardly of the cab (or the sleeper compartment if there is one) for reducing air resistance encountered by a trailer towed by the truck tractor. The air foil system includes: (a) an upright support member including attachment means on its base which is adapted to detachably mount the support member to the tractor frame rearwardly of the cab, the support member having a transverse dimension approximately equal to the width of the cab; (b) an air foil adjustably mounted on top of the support member in a manner such that the air foil is adapted to extend above the cab; and (c) an adjustable side panel mounted on each end of the support member, each side panel extending downwardly from the top of the support member toward the base. Each side panel has a height greater than its width.