You may or may not like GRID Racer if you are more into street racing games. It uses some open wheel cars, but has quite a few trans am style races. The graphics are great and the game play is fun. You should be able to find the demo somewhere on the net to try it out and see if you like it.
I don't have anymore suggestions. That is the only "full install" racing game I have played on the PC other than racing demos. Did you try the latest NFS racing game that was released a short time ago?
I enjoyed the single player mode and I played it more than multiplayer. You have to pick a teammate, sponsors, buy cars that fit classes you would like to race in, customize them etc... I spent a lot of time playing GRID when I first got it as I was really addicted to playing it. If you don't want to play through all of the races to unlock everything, I found a saved game file on the net from someone who had already beaten the game so you can race all of the tracks.