Go to Edit Materials, find your track texture, and change the selection for rFactor material name or racer surface, depending on the game you're building for. I don't know what's needed for other games.
As far I know, there are just little effects of smoke for tarmac, little brown dust for grass and no dust for gravel, speaking of RBR RX. And I see them only during harder spins. Since there's no outside (field) cameras for playback, I don't suffer to much
How it looks like in high resolution (I use 800x600 only - better FPS