If you want to improve the model, please fix your vertices.
Keep the main lines active otherwise when you'll calculate the faces, your car will become a black mass. BTW you can right now click on surfaces > normals > calculate.
Here's some tips (by Burner) :
I dunno if you've read this
tutorial but some pics are evident to know more about the poly placement. Moreover you can check the cars that you converted to MM2 by checking the wireframe of these.