I find this thread a bit strange, because for years I've thought of adding a passenger side mirror to my old car.
When you want to get in the right lane, you are blind to what is racing up behind you.
No passenger side mirror in 1977.
In another thread they said both side mirrors together account for .1 in your Cd, I don't see how removal of one mirror can make a 1-mpg difference. Maybe it prevents you from getting in the fast lane for fear of having to get back over, but that would be about it.
Just my two cents....................safety first.