Those look like they'd be forcing the air pretty far to the sides and creating a lot of high pressure in front of them. While blocking the tires is beneficial, you might want to make those longer and at a softer angle, preferably starting from near the front of your bumper.
Though on the Corvette C6 ZO6, there are wheelwell extensions that are completely perpendicular to the car's surface, as well as a soft, rounded exit for air. There's one at a local dealership here, and I'll see if I can take some pictures of it sometime. For now, here's these:
http://www.bryandickey.com/old/image...p-1280x960.jpg
http://www.theautochannel.com/news/2...27178.1-lg.jpg
You can see the flat deflector in the front in that picture, and the wheelwell inside is curved to meet the side behind the front tire.
http://www.z06vette.com/gallery/imag...1_Image006.jpg
It also looks like they use a hard edge behind the rear tires:
http://www.cargurus.com/images/2007/...pic-31863.jpeg
The Corvette is able to get 22 highway MPG because of it's very low drag.