Abstract: An airfoil member for a land vehicle in particular for fixing to the roof of a cab of a tractor-trailer rig, in which the airfoil member is provided to reduce drag when positioned substantially horizontally for the case where either there is no trailer unit or the height of the trailer unit is lower than that of the cab of the tractor unit. The airfoil member is also movable between a position in which it acts as an air deflector and a position in which it acts as an airfoil. The airfoil member has a pair of side portions and a central portion having a much thinner cross-section in order to reduce its weight. Its height above the roof of the cab can also be adjusted so that it can be set to the best suited position either as an air deflector or an airfoil in order to reduce drag to a minimum.