Car get a lot of dirt around it, like it has just raced Dakar or something. I think it's a good feature, but this is too exagerated. I would suggest reseting car dirt texture in each try in Time Trial (currently it's the same for all the Time Trial session) or when changing the session. I would suggest aswell to minimize the effect because this is not how a car get dirty. The car should be covered of rubber and all, not only sand or mud, which I barely have seen in real life cars.