Check out this site boys.
http://kiwidirt.com/

Tim Collier and I were there last night and downloaded some of their stuff. Very well done. They have SprintCars 1.2 for rFactor (updated from SlimJims original physics) as well as a new Bomber Dirt Car mod you can download. In addition several dirt tracks and some setups. You need to register for their forums to access the downloads but it's good stuff.

Physics feel pretty good to me -- and they are apparently still a work in progress.