Abstract: A rigid liquid storage tank for mounting atop the rear portion of the roof of a tow vehicle and having a configuration whereby during forward vehicle movement relative air currents are deflected by the forwardly facing surfaces of the tank so as to pass upwardly and outwardly around a trailer towed by the tow vehicle to reduce drag forces on the trailer. The length of the tank is more than twice the width or height of the tank and extends across the rear roof portion transversely of the length of the tow vehicle. The tank has an anchoring connection at each of its ends to secure it to the tow vehicle roof at opposite sides of the roof. The cross-section of the tank is trapezoidal throughout its length between its end walls. The tank walls and interior baffles which dampen liquid motion during operation of the tow vehicle are made of plastic which is sufficiently stiff to make the tank walls sustain the shape of the tank during operation of the tow vehicle without need for additional supporting structure.