Some very impressive figures there! I've so far averaged 42.5 mpg over 1800 miles of driving, not quite so good as you guys, but on the other hand I do have a brutal afternoon commute on LA freeways (or sometimes off the freeways because they're just too clogged!)
One observation drawn from my experiences... I sometimes seem to get an encouragingly good figure off an individual tank (eg 48), only to discover that my next tank does less well than average (maybe 38) - the first bar disappears oddly soon on the fuel gauge. What I suspect is happening here is that different fillups will shut off at varying levels of 'fullness' of the tank... different pump, different shutoff detector, different angle of the nozzle, what you had for breakfast that day, etc.
I find it's helpful to keep track of the average as well as the individual tanks... this is easy to do with a spreadsheet program.
Now to work on my 50mpg... :-)
Mike