Just make sure you make her work for her treat. If you just give it to her as a distraction, it'll make the situation worse b/c she'll think, bark = treats.
For car chasing, stand in front of her to block her view. Get her to sit and after she calms down, give her the treat. It might take several days but she'll get it.
Good luck!
ps. I've heard some good things about clicker training as well. Try searching youtube. I'm sure you'll be able to find bunch of helpful videos.
pps. I forgot to ask. What type of harness are you using?