Dragfoiler II, an effective and practical add-on aerodynamic drag reducing system for tractor-trailers, has been developed. Wind tunnel tests with 1/16- and 1/7-scale tractor-trailer models were used to determine empirical design guidelines for the Dragfoiler II's side elevation and planform shapes. Optimum designs for various combinations of tractor roof height and length, trailer height, and tractor-to-trailer gap length gave zero-yaw drag reductions between 30 and 35%. At a yaw angle of 10°, the percentage drag reductions were about half those at 0°. Off-design performance and the effects of trailer side-edge geometry were investigated. Several full-scale Dragfoiler II's are currently undergoing proving ground and commercial fleet evaluation tests.