Pick something of hers that she finds irreplaceable and make essentially the same ultimatum.
You don't have to stick to it - just let it make its point. If she doesn't understand why this is so important to you, then clearly she doesn't know you very well and you have a major communication problem.
My bet, however, is that she may understand exactly how important it is.
The fact that she says she only said yes to your first date because of the car actually works against you, as she might consider it a way for you to get other women. That is, she is semi jealous of the car or what it stands for.
This does sound, in some ways, like what people here have taken to calling a shi+ test. That is, she may be trying to find out if she can boss you around. Can she?
I think it's a legitimate question. How many other things is she going to tell you to do and bully you into it by ending the sentence with "or I'll leave you."