Why?
Your vehicle's aerodynamics affects your fuel economy to varying degrees. While it may not be possible to change the aerodynamics of your vehicle, it is possible to minimize the impact.

Items like cargo containers and ski/bike racks create additional wind resistance that reduce your fuel efficiency. A study in France found that roof mounted accessories cost you between 7 and 35% depending on what you have on top of your vehicle. No matter what the size, it is costing you more!

How do I do this?
Unload and remove cargo containers and racks when not in use. If you have a sunroof, keep it closed at highway speeds to reduce drag. Repair damaged areas or replace missing body parts as they  also increase your car's aerodynamic drag.

If you do have to carry a load on your roof racks, try to use a container that is as aerodynamic as possible or pack the load with round-cornered items in front to decreases the amount of wind resistance. This can make a huge difference in fuel efficiency, especially on a long holiday trip.