To quantify the amount of drag on an automobile, we use a value called the drag coefficient (cd). This number depends on the shape of the automobile , its speed and surface roughness, the density of the air and the nature of the air flow (laminar (smooth) or turbulent).
Forces that influence drag include the air pressure against the automobile, the amount of friction along the sides of the automobile’s body and the relatively negative pressure, or suction, on behind the automobile.