OK, here's a stupid sort of idea to toss into this dicsussion.
One of the first things I noticed about my P5 is that my engine seems to work a lot harder as the fuel tank level drops. When I have a full tank, it is noticeably smoother and more responsive. This is always true of any car I have ever driven, but in the case of my P5 it seems especially noticeable. I have also noticed that when I check my mileage at the point when the gas gauge is half full, it appears I'm getting good mileage. 28-29. But by the time the tank is empty and I refuel the total mileage is much less 23-25.
For those of you who have been getting better and/or worse mileage than expected do you normally keep your tank more than half full or less than half full? In theory, this shouldn't make a difference, but in reality we may have some sort of fuel pump situation going on in which the fuel system operates more efficiently at higher tank levels. Am I nuts??