Am I in the wrong?

So my bf moms car broke down a few weeks ago and she’s been using our car. My bf and I really don’t go anywhere unless it’s for work or go to the store. I work part time so I don’t work everyday and my bf has football practice which is Monday thru Friday. His mom uses our car everyday in the morning. When my bf and I were the only ones using the car , our gas would last longer and now that his mom uses it also the gas only lasts about a week. She doesn’t put in gas and I’m kinda annoyed because she mostly uses our car. I feel like if you’re going to use someone’s car period you should at least pitch in gas. Even it’s $5 I mean I don’t expect much but if you’re gonna use our car at least pitch in. Is it wrong of me to expect that out of her ?