Guys, try a 4412 or 4415 inverted airfoil. You could also try a inverted 23012 for a race car im assuming that you may wish to have a lot of "negative lift" to push the rear of the car down for better traction and handling correct?
I'm not a race car person in the least bit, and my designs are for helicopter rotor heads etc. so i have almost no knowledge of race cars or the difference between formulas etc. But placement may be a key factor as well.
If you want something to help with traction? then if it were me, id go the opposite route and look for one that had good thickness, say 12%, and a high moment coefficient and use that moment to apply the inverted pressures over the rear axles, or even over the front, almost no one does this since its the opposite in what youre taught.
So give a look at the 4412, 4415, 6415 airfoils. also remember that you can merge some of these together to create a hybrid airfoil if need be. I think the 6415 airfoil should work for what you want.