I was told this story many years ago. I do not remember it exactly, but it is called "Grandma's Gas."
When Tom and Pearl Callen were young, I don't remember if they were dating or had been married a short while, they would often run out of gas. Finally, Pearl's mother made a deal with Tom. She would buy a half tank of gas for his car, but she wouldn't give it to him, and he wasn't to use it. As soon as the fuel gauge showed a half tank, he was to fill up. After that, the second half tank was Grandma's gas. Tom and Pearl's children grew up knowing that they weren't allowed to use it, and I learned from my husband that I wasn't to use it, either.
Just the other day, I was giving a ride to a relative, one of Grandma's great-grandchildren. He had given me some money to pay for fuel, and I commented that I needed to fill up. He looked at my gas gauge and told me I had half a tank, implying that I could let it go longer. But I told him that I couldn't do that, because it was forbidden to drive on Grandma's gas. He smiled, and said OK. It is still true, and he knows it, that even though Grandma has been gone for a number of years, Grandma's gas is hers!