On my car the back end always seemed loose at 135 mph
but one day I had a 20mph quartering head wind and
when I hit 135 the back lifted right off the ground...
I ended up in the other lane just missing the other car
so I put it in the trailer... come Monday I went to
talk to the wind tunnel guys and they showed me some
smoke footage of my body style... the air came off the
roof line and dropped off the back end about 6"-8"
behind the car... all of the area behind the roof
line was a negative pressure and would try to lift
the back end... after seeing that I went to the wing...
I tried the spoiler style first with a adjustable
upper so I could make it taller... that worked ok but
the flatter longer wing worked better with less drag