This practical joke is hearsay. A fellow student some years ago related the following.

This practical joke is hearsay. A fellow student some years ago
related the following. Apparently another student was a bit of
a braggar. His favourite topic was his car, and one sub-topic
was the terrific gas milage (pre-metric) it got.

So it began one evening. Fill up a one-gallon container of gas
each night and pour it into the victim's gas tank. Wait for the
story each day to get better and better. Repeat until it cannot
be taken any more. I believe 2 weeks was sufficient.

Finally the moment (days) of truth. Each night for 2 weeks,
the effect was reversed, and one gallon of gas was REMOVED from
the victim's tank. It was amazingly effective at reducing some
of the stories. I suspect the truth was never revealed to the
victim.