Obviously your dog has such an obsession that she has reached the point where she is completely oblivious to her surroundings, she just wants to chase the car! For these kind of cases, where dogs reach that fixated state, owners need to assert their dominance via touch. This can be done by a simple but firm and confident touch with your hand to the neck. It isn't hurting the dog in any way, it is just like someone touching you on the shoulder to get your attention. It is a primal instinct. Dogs will often nip other dogs to tell them 'I disagree with that behaviour'.

If you do this correctly, the dog should recognise your existence then, as you are knocking him out of his fixated mental state. If this doesn't work it could mean that either your not doing it firmly enough to match the level of his minds fixation, or your not promoting the right energy- you need to be calm at all times, never angry/frustrated etc., but please consult a professional before hand.

Hope this helps!