it depends if you want your background to be out of focus or not. And how close or far away from the subject you are. I'm going to say you'll probably want most of the background to be out of focus, so the viewer seeing the car and model, they'll stand out against the background.
I wouldn't go extreme, try F9-F11 and see how you like that. See if it's not an extreme close up shot, and you want to a good portion, you don't want to totally blur out the background because you kindda wanna show the scene you're working with to make the picture what it is. I don't konw if that's a good explanation? But yeah give that a try see how you like it and for close up shots where it's part of her and part of the car then you can have more blurred backdrop.
for me its going to depend on wether you have a good background. allot of times when im shooting a car. i want a simple background just so the car and girl are the focus and you dont lose site of them looking around at the background. as somone mentioned again your going to want a deep enough dof to get the model and the car in the shot. thats going to dictate allot of times how much the background is out of focus or not. if its your first time just figure out poses and positions ahead of time esp if she has never modeled before. (something ive failed to do and it showed) make sure you get to know the model so she is comfortable with you and she is relaxed. and just have fun.
Which is hotter--the car, or the model? If the model is hotter and she is the subject, shallower depth of field can work quite well, shooting from 15-30 feet with a long lens like a 135,180, or 70-200, or 300mm prime and using the car as a prop for her. In such cases, usually not that much of the car is shown.
If the car is hotter, it's common to use smaller f/stops and somewhat shorter focal lengths, to pull deep depth of field and to show off the car. You can of course, feel free t mix n' match. Pick up a few copies of car mags, to see the various ways to shoot this kinda' stuff.
Women and cars go together like cheese n crackers...they are both fantastic on their own, or paired up.