Thanks for publishing this
. I am going to try mirrors off, and try to break the invisible barrier of 50.6 (UK) MPG - no matter what I do, I can't seem to get past the number!.
Regarding the drag of the mirrors, it has already been mentioned that they add to frontal area, and are placed in a position where the air speeds up due to being forced around the car.
There is also a long column of turbulence behind the mirrors - a few days ago, as a passenger, I held a cup of steaming hot tea (from a thermos) behind the mirror, and the turbulence was very well demonstrated!.