Somebody been sneaking around at night and adding gas to your tank!!!

Years ago, we actually pulled that old trick on a friend who bragged so much about his V-Dub's gas mileage. We added a little bit to his tank for a couple weeks in a row; and he was really proud of the MPG. Then, we siphoned a little out for the next couple weeks - drove him crazy.
Wasn't college fun, so much time, so little to do.

My big ol' 300SE with the 3.45 diff will get about 18 MPG on the highway doing 70, and about 18 around town. And about 18 sitting in the garage, it seems. Sigh.