Thank you for starting this thread, cause I was already thinking about this. I also start to notice that when I load the car (or just wife and kids, not that they are so heavy but still
) and make a long highway trip, that I'm roughly getting the same consumption figures than with an empty (and stripped) car. Difference is often +500 lbs, but also a 2 cm drop in the front and 6 cm or more in the rear.
That is to say, the roof line with an empty car is quite horizontal, but when it's loaded it gets that typical streamlined downward slope.
It will coast better due to the extra weight, but there might also be the improved air restistance thanks to a better shape. And it's not only the air that passes over the car, but also less under the car and in the wheel arches ...