Using the
aero/rr web tool I made based on Dan's spreadsheet, my car theoretically gets
56.7 mpg (US) @55 mph (Cd: 0.34, frontal area sq.M: 1.858, @ 25C, default fuel & engine eff. values). That's actually not far off my actual FE at that steady state speed.
Removing one mirror: Theoretically, using the more conservative GM numbers, taking off a mirror reduces the Cd by 0.015 and the frontal area by about 0.0375 sq. M.
That improves my 55 mph highway FE to
59.64 mpg (US), or +2.94 mpg (US) / +5.19%.
That's a gigantic improvement in my books, if it's true.
EDIT- I mistakenly used the least conservative numbers. The Chalmers report has the more conservative range. So I'm guessing the actual effect will be less than 5.19%