i thought i'd discovered something interesing when i noticed some small flaps portruding from underneath an opel corsa C and later saw the same thing on an opel astra G... they where about halfway the car and a little wider than the tires, with a gap in the middle, not all cars have these. i asked my dad about them and he said they where stone deflectors to protect the fueltank and some lines running there from being damaged by stones kicked up by the front wheels...
suddenly i remembered i'd seen something similar but much bigger on some old saab 900. and it would make sense they do much the same, as scandinavian countries have many gravle roads, but that doesn't stop them from taking most of the saabs turbo.
in fact my dad tolled me that when two cars pass eachother on such a road, in stead of moveing to the side of the road to give way they move to the middle and pass eachother very close, to have all stones that are kicked up bouncing of the underside and not the body of the cars.
to bad, but it shows you shouldn't just copy everything that looks like it might be for aero without doing some research. most likely these things will actually add dragg.