Good attempt, but there is much left to be done here. As mentioned the dirt looks more like fog, in reality there should be more of an obvious spray coming from the tires. The car doesn't really feel like it should be on this bg. I think the shadow plays a large part in this, also it sort of feels like if that car was coming around that corner, the front wheel would be pointed towards us as if it were drifting around that corner.
I would google image search WRC and you will gets lots of pics of cars in this atmosphere. And from there you can try to c+p the dirt patterns or try to recreate what you see happening in those pics. Keep at it though.
