My car has a rear bumper which has a lot of empty space inside and is not closed off at the bottom, so I thought I'd try covering some of the gap at the bottom, a kind of partial rear undertray, with some plastic sheet which I screwed to the lip of the bumper and fixed in place.
I didn't cover all the gap, but just what I could do in one go.
My mpg went down from 40 to 38 or thereabouts, recorded over several months.
I recently removed the piece of plastic to see if anything happened. My mpg went back up to 40.
I'm guessing that the
partial covering of the gap created more drag instead of reducing it. (there may have been other factors, like the weather, but I think everything else was consistent)
I suppose the moral of this tale is, don't do anything by halves! And this A-B-A testing that I've read about is a good way to prove a point one way or the other.
