Should I be mad?

So I'm 21 living at my parents house and I have a 4 month old. 
I have a car. That I bought MYSELF. For some stupid reason I let my mom (and stepdad) put my spare key on their key ring. I find out sometimes that they take my car without my permission because they "have no gas" or "their truck is too cold to start" (we live in Alaska)
My car is covered in dog hair now because they constantly are taking it. I'm really annoyed. But I don't know if I'm wrong for feeling this way. My mom is a narcissist so if I tell her not to take my car without asking I am almost positive she will flip shit on me. 
So basically I can't win either way. 
My car also gets crappy gas mileage so them using my gas just to not use theirs.. Is kind of bugging me even more. And the more I think about this, the more angry I get.  
DISCLAIMER***** they do NOT pay for my gas