Probably trying to live frugal and one of the ways she's doing it is by not using gas/putting miles on her car. Which frugalist is a fine lifestyle when you're not grifting as impoverished and guilt tripping others to take on your expenses, but often that's what happens. There's a reason these fringe lifestyle individuals tend to have few, if any, friends. Not to be an armchair psychologist, but I think many of these people are probably on the OCD spectrum and do not feel "pure" unless they're adhering perfectly to their extremist compulsions to not make waste (such as using a little gas, putting a smidge of wear on her tires). This is why she can't do it even every once in a while when it'd save her a lot of trouble. Unfortunately, what's going on in another adult's psyche is not OP's battle to wage, though.