My longest tank is 477 miles on my 11 mile back-road commute at 35-45 MPH, 39.5 MPG, took it down 10-15 miles below E. It was April 2004, no A/C.
My best fuel economy for a tank is 43.7 MPG on a 300 mile round-trip on state highways mostly at 50-60 MPH. It was 90 F with the A/C on low both ways. I repeated this same trip a few weeks later, and the weather and A/C were the same (maybe it was 93 F), and I got 43.6 MPG for that trip.
I've found that the tank takes much more or less fuel depending on season and fill rate. At a slow pump (or if I don't pull the trigger as far) in the summer I can go 130 miles before the gauge gets back down to F. At a fast pump it is usually 55-75 miles down to F. That's more than a 2 gallon difference! If you follow a good fill with a bad fill, your fuel economy will look GREAT, and vice versa it will look TERRIBLE. So, to get more accurate fuel economy calculations, besides writing down the mileage when I fill up, I also have a column for the mileage when the gauge gets down to point exactly at F, and THAT'S the number I type in for my mileage calculations.
Another way you can get better numbers is to average over 2 or 3 tanks.
Here is an almost blank excel spreadsheet template I use for my mileage calculations:
http://home.comcast.net/~ksbrace/Ref...e_template.xls
You fill in your fuel purchases and it graphs 1, 2, and 3-tank average fuel economy for you. You are welcome to download it and use it.