I remember hearing a TV celebrity (I cannot remember who) telling the story of how him and two mates all lived in the same cul-de-sac, and one bought a brand new car. It was back in the days before locking fuel caps were standard. The new car owner, like me, was OCD about his fuel figures and, like me, boasted about them at the least opportunity. What the celeb and the other mate did was pour an extra gallon in, once a week or so, for the best part of the first year. Then they stopped. The poor owner suddenly found his mpg tumbled, and repeated visits to the dealer did nothing to solve the issue. Eventually, for the dealers sake, they told him what they had been doing!