A light-weight cargo-protecting and air-flow-drag-reducing wind-deflector is made quickly and easily attachable to the roofs of automobiles or truck cabs by suction cups and conventional gutter-anchorable straps. The wind-deflector is formed of a curved fiber-reinforced plastic sheet and is made adjustable about a transverse horizontal axis close to the vehicle roof at a fore-aft location selected for optimal effectiveness. The slip-stream-modifying angle adjustability is provided simply by a pair of arms medially divided and slightly overlapped, which arms laterally connect the rear portions of a rectangular base frame to the upper side portions of the plastic sheet. The arms are made effectively length-adjustable by changing the angle defined at the interconnections between their divided sections. The overlapped mating faces of the divided arms are preferably radially die-grooved about bearing apertures which co-axially receive the ends of a single axle-rod. A rigid tube surrounds the axle-rod and co-operates with a lock nut on one end of the axle-rod and a wing nut on the other end to clamp the divided arms in their selected die-groove-fixed positions. The wind-deflector is flatwise collapsible for carrying, for storage, or for minimizing air-flow drag during nonuse roof-attachment.