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They may not work in that placement on a vette... Especially without a huge rear wing, but on cars like the Evo and Sti that have functional rear spoilers, wind tunnel testing confirms that when placed correctly they have a positive effect on high speed stability and downforce. They also work to reduce drag by "breaking up" the air as it travels over the roof edge and over the rear of the car.
I looked at these just for laughs. But then I read the description and the claims made about these things are even more funny:
How the hell can you decrease drag by creating more points for air to interact with??? That doesn't make any sense.
There will be a small increase in drag from the fins themselves, but there will be a greater decrease in drag from the delayed airflow separation. Same concept used in aircraft. The separation of airflow will increase the car's ability to generate downforce. Always do a little research before posting your disbeliefs...
As FYRARMS mentioned they do indeed work and are used on aircraft as well as some cars. They are used to control turbulance and the boundary layer, they're also known as vortex generators. Same general concept as to why golf ***** have dimples. You may also hear people say the flow is "tricked".
Anyway, the C5 coupe (with headlights down) is one of the most aerodynamic cars out there with an incredibly low coefficient of drag. I'd say these are not necessary. The shape of the hatchback is great already so no need for these. But if they tickle your fancy they will most likely do no harm either.