I had near perfect weather conditions for doing a little experiment: light north winds, which are blocked by a ridge and tree line that runs along the north side of my usual "test" road, leaving the road in nearly still air.
I did an
A-B-A, where
A = both mirrors on, and
B = passenger mirror off (with the hole covered) and driver's mirror folded, with my aero hinge insert in place (see pics, earlier in this thread).
Speed was 88 km/h / 54.7 mph, cruise control set once, cancelled with the brake and resumed for each pass. (Yes, an odd speed choice. I tried to set the cruise control right at 90k, but didn't quite make it.
)
55.25 mpg (US) - average of first
A runs (2 bi-directional runs)
56.51 mpg (US) - average of
B runs (2 bi-directional runs)
55.04 mpg (US) - average of last
A runs (1 bi-directional run)
For an average difference of
1.27 mpg between all A and B runs, or an increase in FE of
2.3% at this speed, for this car. Not quite the 2.94 mpg 5.19% improvement estimated earlier in this thread (for just one mirror removed!). But also not insignificant.
So there you go.