I don't think mirrors afftect your car that much. If you hold your arm out the window doing 60. How much air force is pushing back on your hand maybee 5-10pounds. But if your a FE fanatic why not take it off.
2008 EPA adjusted:
Distance traveled by bicycle in 2007= 1,830ish miles
Average commute speed=25mph (yes, that's in a car)