Fender skirts are part of a vehicle's body that cover the upper part of the vehicle's rear tires. Vehicle manufacturers use fender skirts to increase the vehicle's aerodynamics. Fender skirts cause air to flow over the vehicle's body rather than being trapped in the rear wheel well.
The Nash Rambler experimented with front fender skirts from 1950 until 1954, but found front fender skirts impractical because the front tires must turn to steer. The City of Los Angeles uses rear fender skirts on its city buses to keep items on the road from rolling under bus tires.
Many fender skirts are detachable to allow drivers to change tires or install tire chains. Detachable fender skirts are hard to keep in place and add cost to each vehicle. Fender skirts also increase pressure on rear tires, which increases the chance of the rear tires failing.