This might just not be possible due to how the terrain is made in the game, but i think it would feed a lot to the realism of the car if, like in real life, softer ground gives less feedback of the terrain and less bumpyness to the wheels. I noticed that when driving over sand or grass that a small bump was felt the same than if it was over concrete, which isn't quite realistic. Dirt is quite soft and it helps dampens some of the car's motion when taking bumps or moving in it.
