And also imagine you raced the whole campaign on Physics A and got used to it. But then you wanna join a race but the host only uses Physics B. You loose every race because you're not used to this kind of physics. Nah really, too much confusion. I'd go with one physic system for the whole game. Like I said, Wipeout, Flatout, Destruction Derby... almost all classic games only have one physics system and the people who liked the game got used to it.