My apologies if the following is too basic, but from the sounds of it you might want to review this hard surface modeling tutorial (http://vimeo.com/10941211
) before starting on your car. This tutorial doesn't show how to model anything in particular but it does cover best practice for creating and detailing all sorts of hard surfaces while avoiding mesh tension errors when subdivision is on. From past mistakes I've made modeling cars, I would pay attention to the following two things:
1) Use only as many polys as you need. Additional verts lead to additional headaches, since it becomes increasingly likely that one of them is out of position and is causing ugly distortions in the smoothed mesh.
2) Constantly turn on and off your NURMS subdivision (have it a shortcut like the tutorial recommends). This is how you will determine if you have just enough polys to create the form you need. It also lets you see if any verts or edge loops are causing unwanted tension or distortion when your model is subdivided.
These things seem basic, but it's pretty easy to forget them when you are caught up making something complicated like a car. Also, I think the principles of creating hard surface models are poorly covered in many tutorials so it's helpful to have them fresh in your memory before starting your model.
Best of luck with the modeling! Hope to see your progress on here soon.