Funny story:
When I was in my teens my mother went out with a guy called Robert who lived in Motherwell. Robert's nextdoor neighbour bought a new car and was bragging about it something awful, getting on people's nerves. One of the things he boasted about was the great economy it was supposed to give.
Robert decided to play a trick on the guy. Every week he hopped over the fence and put extra petrol in the guy's car! This was back in the days when most cars didn't have locking fuel-caps. He kept this up for...I can't remember but say a month or so and sure enough, the guy was crowing like a cockerel about the amazing MPG he was getting.
Then, Robert started visiting the car but instead of adding fuel, he siphoned out the petrol he's been putting in! :0) His neighbour was totally confused and furious. He drove the dealer mad demanding to know what was wrong with the car. The dealer of course could find nothing wrong with it, and couldn't figure out how it could possibly have been returning the epic economy he claimed of it.
I remember that Robert did tell him what he'd done but I can't remember what the final outcome was, but the whole thing was pretty funny.