This is a decent setup tool that I base most of my tunes on. Its for F3 but the end results are the same.
http://www.forzatraders.com/forza-mo...car-setup-tool
+1 on keeping only "tuned" cars in the the Shared Garage, preferably maxed out for their specific class. It's great to have loads of cars but something that is halfway between two classes is not going to be very competitive online, and that's really the only thing they can be used for.