I think for a little over a year now my girlfriend thinks I own a car
Did she tell you that she thinks this?
She does? That is what I want her to think.
It may not be a great idea for you to be dishonest to your girlfriend. If she finds out she will have trouble trusting you down the line.
She wouldn't be happy if I told her the truth. Plus, shouldn't she know better?
Lying to your partner is a bad habit to get into. Trust is super important when it comes to being in a relationship long term.