What many kids don't believe, until they accidentally or purposely go without their "nicotine fix" for a while is how addictive cigarettes are and how badly they are physically hooked. Since I'm a believer in the discipline of natural consequences, I don't believe the taking away of her car is a natural consequence of her continuing to smoke. Yellow teeth, stinking clothes, horrible breath, shortness of breath, lots of hard-earned money down the drain, etc. -- these are natural consequences of continued smoking to a teen. A majority of girls I have counseled smoke to suppress their appetite in an effort to diet and/or remain thin. It works, so it's tough for them to give up this "diet aid".
I would say you can calmly state your reasons why you are worried about her smoking, offer to get her any professional medical help she wants to kick the habit, let her know you know how tough it is to kick this habit and that you will do anything you can to help her kick it. I would say that you cannot, in good conscience, give her an allowance or spending money knowing that money may go towards injuring her health. If she wants to buy cigarettes she'll have to do it with money she earns from sources other than you. You can also forbid her to smoke in your house, your car, around her siblings and feel that is a reasonable position. Good luck. This is the habit that is the toughest physical addiction I treat.