http://www.gerrelt.nl/section-aerodynamics/aerodynamics-vitaloni-mirror.html
I won't do this for gas mileage because I only drive about 2K miles/year; but I would if I was going to show the car.
The Vitaloni mirror is/was only about $15 and the mod is essentially free, it's smaller and more aerodynamic. I could add a right hand mirror so I can see the right rear bumper and tire when I'm backing up with little or no penalty. But it would fold up or down instead of back.
It would be quieter. Note also the
raingutter filler.