I think the best comparison for free roam would have to be rigs of rods, which is a free game to get, has heaps of content, rock crawlers, 4x4's ect and the phisics are the best available now.

Even has flexable tires "they bag out whe rock crawling"

and ofcourse,,,,, it has free roam..... This is pretty much the only mode in the game and have a look at how manny downloads they have!!!!!

Proof is in the numbers 1c!!!