They are made of a wall of individual aluminum slats, covered with soft vinyl. In the unrolled position, these aluminium slats form a hard aluminium shell, which not only covers the cargo inside the bed, but also may support loads up 400 pounds on top of it. Another style of truck bed tonneau cover is a retractable unit, which is mounted at the front and sides of the bed and rolls up or retracts from the tailgate towards the cab. The retractable tonneau is typically made of vinyl, plastic or aluminium. Retractable tonneaus are more secure than soft tonneau covers, since they usually lock and are made from a harder composition, but they take more time to install and are designed for semi-permanent installation. Fiberglass, hard plastic, or aluminium tonneau covers are also common. Some may be painted to match the truck, are solid in construction, and can be locked. These covers are usually heavy and require gas struts to assist in opening and closing. They operate much like a vehicle's hood, typically opening from the tailgate end of the bed (back to front). Some have multiple compartments that open front to back, back to front, side to side, or even rise vertically. Fiberglass, hard plastic, or aluminium tonneau covers are sometimes installed as a factory option on new vehicles. Many sellers claim that tonneau covers improve gas mileage because they make the truck more streamlined. However, air currents create a wake inside the pickup bed, which helps the aerodynamics. A tonneau cover interferes with this wake, and scientific tests have shown little to no improvement in mileage by using a tonneau cover traveling at less than . A similar effect is seen when the tailgate is down and the mileage goes down. Race cars Early open-bodied touring-type automobiles used tonneau covers to protect unoccupied rear seats. As early as the 1930s, lakes racers, searching for an extra competitive edge, imitated early automobile construction and skinned the cockpits of their roadsters and streamliners with removable canvas. The skins covered gaping cockpits that would otherwise disturb airflow and create undue drag; as a result, tonneau-equipped cars ultimately went faster with a given amount of power. Sports cars Tonneau covers are available for open sports cars such as the Porsche Boxster, MG, Triumph, and Austin-Healey. These covers, often made of natural or artificial leather, cover the entire passenger compartment, and are zippered