Car body. Computer-aided design (CAD) of a car body. The car body is shown as a wire-frame model. The design of a car body has to take into account factors such as air flow over it, driver and passenger comfort and the driver's field of vision. CAD is a tool by which objects can be completely designed in three dimensions, and tested before they are built. It saves time and money, as incorrect designs can be corrected quickly and easily. The data from the computer can be fed into a mechanical production line, and the object can be automatically built to the specifications.