For a long time, the coefficient of air penetration of cars (Cx) obsesses manufacturers. That’s aerodynamics.
How to improve aerodynamics ?
But just a bike rack, a roof rack (even unloaded) or a roof box yet streamlined to ruin their efforts. Whenever possible, try to get all your luggage in the trunk. And if the roof box or gallery are essential, remove as soon as they are no longer needed … and save up to 15% fuel.