Correct.It's not implemented yet. If I understood correctly it should come in one of the next updates as it was one of the feature of the crowd funding campaign (have a look at Update 5 in the first post).
The effect is not only on graphicsyou dont see the dirt anyway in closed cockpit cars so its useless to me
Even if you're not an open-wheel driver, you'll still have to deal with the effects of dirt on your tires.you dont see the dirt anyway in closed cockpit cars so its useless to me