The FX are really excellent.
But I have one concern that will end up sounding like a science lesson. Hashlister mentioned that the smoke seems to be rising too fast...
The smoke should be following the exact same path as the flames. This is because they are both the same thing...except at different temperatures...as the ash is expelled off the wreck, it's freaking hot, causing it to glow (the fire)...but as it cools off it turns from yellow-red, then to black....the black being the smoke.
But not all flames produce smoke... (because the hot products are water/carbon dioxide)...
I hope this helps... You are definitely on the right track with your FX...
Shane