I would figure out the current blue book value on the car, and then tell her you would be happy to sign the title over if she buys you out. If it is fully paid off she should be able to get a loan for half the value of the car. Otherwise you intend to stay on the title so that when she sells it/trades it in you get 50% of the value at that time.
My W and I will be headed for the same thing, though I don't think she realizes it. On my car we owe $6,000 and it's currently worth $4,000. On her car we owe $7,000 and it's currently worth $15,000. So hmm... do I intend to simply let her walk away with $8,000 equity while I get left underwater by $2,000? Nope.
Married 6 together 8 Me:38 W:31 second marriage for both SS12, SD10, S6 Bomb: 9/8/11 (day before our 5 yr ann) W moved out: 2/18/12 D final: 11/12/12 Share S 50/50. Spend as much time as I can with SS & SD