You get a medal for level of completion, potentially 3 different cars you get iirc
Also, for tires, you should be able to see all (11?) types of tires, but if you can't install it there will be an exclamation point next to it...I don't know if you are in the right section.
Boucher apparently there are more than a few of those types of awards, I think I read that at 50 cars owned you get a random vehicle...some people have gotten badass cars, most have gotten crap.
You also get a reward for taking to a track 100 times
I have kept a lot of my prize cars, holding them for now until I feel I don't need/want them anymore.
I just won the Ferrari California, probably gonna sell that one though.