L.W.
My parents rule was this, if you are driving, you fill it up. If a quarter tank or more was gone, I had to fill up on my way home. That way, whoever got it next had a full tank. I could see some people abusing this, but my brother and I never had a problem. Neither one of us drove it regularly enough to always be the one filling it. As far as maintenance, it was pretty much the same. If it got messy when one of us was using it, we had to clean it. Tune ups and such were paid for by my parents. When I did finally have a car of my own, my parents bought the car and everything else was on me. My brother and I never fought about it, but I get the impression we might be unusual.