Truly low drag vehicles have totally enclosed wheels.
Obviously,if you have a front engine car with front radiator,the air has got to have a pathway out.
Dr.Alberto Morelli has invested the most time of anyone I've read of to design a minimum drag cooling system at the Pininfarina wind tunnel with his CNR 'banana' car of 1978.
None of his air extractors were at the wheel houses.
The side extractors on the 1982 Pontiac Trans Am Firebird added drag.It could cost you hundreds of thousands of dollars to scientifically design these critters.And they would be designed on a case-specific basis,so what worked on one car would not necessarily benefit any different car.
I would recommend you take a cue from basjoos' AeroCivic.
http://asset1.cbsistatic.com/cnwk.1d...ide550x412.jpg