Back in the late 70s, early 80s I worked with a guy who bought a Volkswagon Rabbit. He would come in every day and brag about his mileage (30 mpg, according to him). So we decided to have some fun.
Every day someone would go out and add about a 1/2 gallon of gas to his tank. He soon started talking about how he's getting mileage
way better than he's supposed to, almost 45 mpg.

We let this go on for about a month or so.
We then started going outside at lunch time and syphoning some gas from his tank. What's going on. I'm only getting 15 mpg.

He even went back to the dealership, but they could find nothing wrong.
Eventually we felt sorry for him and his mileage went back to normal.