Dread,
Catching up on posts, the first message here where the guy reports spikes of fuel efficiency, reminds me of a story from a friend at a vehicle testing facility several years ago. One of his co-workers bought a new Volkswagon, and was bragging about the great fuel economy. He measured it every tankful. So these guys schemed a plan to play a practical joke on him by adding gas to his tank every week. They had him really going, after a few weeks; his gas economy kept getting better and better, to the point where it was almost double where he had began. They had to finally let him in on the scam... One of the few reasons why you could possibly get a lot better miles per gallon rather than poorer mpg.