Firstly, I like your ideas. But I think they could have been a better execution of them. I can see some problems.
- The front wheels on the '86. The effect you've added doesn't look that realistic.
- The smoke on the rear wheels doesn't look realistic either. It wouldn't go around the way it does on this image. Try using references to make it look more realistic.
- The way the bumper on the '86 is scraping the ground, I don't think that the sparks would be produced as it's not metal.. But I like where you were going with it. And I don't think they would be going in that direction, straight along the ground..
- The bonnet on the '86 isn't looking too bad, but I wouldn't have put it an an angle like that. And with the blur it makes it look kind of fake.
- The reflections underneath the cars is too harsh. Look at some references to try and get an idea of what they should look like.
- Underneath the 4x4, there is the previous background that hasn't been removed. It sticks out like a sore thumb.
- The car in the background, that's hit the mountains face, doesn't look like it fits. It's way to big. It would be alot smaller.
I didn't mean to nit pick, but I am pointing out areas on which you can improve on. I hope that the information and advice I gave you will help you to develop your level.