You should not delete the contents of the cache folder. Reason is, if there is an rFactor 2 update, you will have to re-download all of the contents of the cache folder again.
My only recommendation there would be to delete any replays you have saved... thats about all you can do, other than deleting specific cars/tracks. Make sure you dont have multiple versions of the same tracks, I noticed at one point I had 3 different Bahrain tracks, and all were practically the same. That saved half a gigabyte on its own. Good luck