Cars are designed in wind tunnels to offer the best drag coefficient taking into account basic safety and mechanical packaging requirements. Any impediment to the smooth airflow over the body will increase the drag and the car will require more power and fuel to overcome it. As a point of interest, the latest G30 5-Series BMW
is currently one of the most aerodynamically efficient cars, with a 0.22 Cd drag coefficient factor. Roof racks should be removed when not being used and driving around with a trailer when it is not needed should be avoided.