The back bumper is not really anything great I already had it from the convertible project so I stuck it on this car. I can't remember the exact numbers but on my coastdown hill I think I went from a peak speed of 75mph to 78. About the same difference in speed as rolling down a window without the wing windows open. So it isn't much really. Cutting the section of the bumper that hangs below the rest of the underside would be much better but swapping covers was much easier and quicker and leaves me a good cover to put on it when I sell it. Next time I get the car in the air I will take a picture of the underside so you guys can see how far down the bumper hangs into the wind. I figure most cars have a decent shape to their bumpers and ventilating it won't help but not every car is the same. Now if I was drilling holes in the bumper of my Volt you guys could rightly call me goofy
The mini skirts are an afterthought. I had a strip of plastic from cutting the back and was only that thick. So I figured why not and slapped them on. If they don't work I can always make a larger set. I am not out any material since it was just scrap pieces anyway. One day in I will be driving in the rain and stick a camera out the window and see if they work, if not I will make bigger ones.
I just took the car out for a quick drive and it seems like it coasts much better than before. If they were not patrolling heavy where I normally do my coast down test I would probably go out and try it this week. I don't think they will buy my excuse doing over 80mph in a 65 of just testing my car out