well, screwed up the undertray test today.
i got in two bi-directional runs (of a planned three) with the tray on, when at the end of the 2nd run I heard some cardboard dragging. the rearmost panel had come loose and was hanging down across the width of the car "scooping" up air.
i don't know how long it was detatched since i didn't peek under the car from the time i left the house until the end of the 2nd run when i heard it hitting the ground.
so i pulled all the undertray off, hoping the rear panel only came undone at the end of the last run and that the full set of undertray runs wasn't affected.
i did 2 bi-dir "naked" runs for comparison, which produced very similar numbers:
52.93 mpg avg with the undertray (2 bi-dir runs)
52.38 mpg avg no tray (2 bi-dir runs)
speed was 55 mph.
Conditions at SUPN6 as of (1:00 pm EDT) 1700 GMT on 04/06/2006:
Wind Direction (WDIR): SSW
Wind Speed (WSPD): 6 kts
Wind Gust (GST): 7 kts
Atmospheric Pressure (PRES): 29.87 in
Air Temperature (ATMP): 41.2 °F
all of which means either the undertray has a very small effect on my car, or it was hanging down on both bi-dir runs.
of course, the only way i'll know for sure is to do it again
crap.