Many years ago, in a tongue-in-cheek manner, Mechanics Illustrated or one of the other similar magazines (don't remember which, but it was long before the internt) purchased a variety of gadgets that guaranteed increased mileage by various percentages and installed them all on the same car. The percentages added up to something like 115%.

As a result they had to stop the car every 200 miles to drain some of the gas out of the tank. "Just kidding" they said. "Milage actually fell off by 15%."