This song is about the vicious cycle of life. The vicious cycle of poverty, of alcoholism, of single parenthood. The fast car symbolizes her dreams of making it out of this cycle.
Sadly it never happens. She ends up back in the same position she started, with nothing; the same job, poor, taking care of an alcoholic partner. She realizes that her dreams of getting out of this lifestyle are never going to happen and now she wants him to leave and take his fast car (all the broken promises and dreams) with him.
According to Chapman herself, that is exactly what the song is about.