Abstract: A truck tractor-trailer assembly having reduced aerodynamic drag. The skins of the tractor cab and trailer extend longitudinally toward each other and terminate in opposed trailing and leading edges which are as close together as possible while still permitting a full range of yaw and upward pitch about a fifth wheel assembly coupling the trailer to the tractor. In addition to accommodating yaw and upward pitch, the tractor-trailer skin edges are separated by a minimum clearance distance determined by manufacturing tolerances, plus clearances in the fifth wheel assembly, plus elastic deformation of components, plus the range of maximum roll of the tractor frame. An optimal construction employing concentric spherical surfaces is shown, as are embodiments having minimum skin and volume gaps meeting other design criteria. A method of reducing aerodynamic drag by reducing tractor-trailer skin and volume gaps also is described.