Just let things happen naturally. She was the one who told you to be platonic, so in my mind it's up to her if she wants to move in a different direction. It doesn't sound like you "must have her," so I wouldn't risk making things awkward by asking her/making a move right now.

Girls will totally just hang out in the car just to talk. They operate on a different wavelength with those types of signals than men do. I think she just sees you as a cool guy who is fun to talk to. If she is interested, she'll let you know.