Just turned 100k on my 08 5AT and got 31.6mpg on the trip with AC on the whole time with 3 people aboard. Still on the original shocks/struts and brakes too.
2010 Sport, 5AT - Had a bad week I guess. Almost all city driving and in a hurry everywhere I went. I got 16.8mpg on that tank. Ouch!!!
I am with you on the tank level, the last half of the tank on my 2010 5 goes much quicker than the first half. On my 2012 3 I notice the same thing. I recently took a road trip in the 3 and wanted to test my theory. I topped my tank off until gas starting coming out. I then drove it until it was at half of a tank. I stop and filled the tank up and topped it off the same way I did the first time. When I calculated my mpg's I had 49 mpg's highway. I then drove until the gas light came on at empty and then filled up again. When I calculated the mpg's for the whole tank instead of half, I got 45 mpg's highway. That was a 4 mpg swing from the first half of the tank to the last half of the tank.
All I am trying to say is that the fuel gauge isn't accurate. The first half of the tank last longer than the last half of the tank, according to the fuel gauge. I know people who will calculate their mpg's by computing what they got on the first half of their tank, instead of their entire tank. This method skews the mpg's. Basically my point is that there has to be a consistent method for calculating mpg's. Regardless of if you top off or stop when the pump clicks, calculating your mpg's across the full tank, instead of half the tank or 3/4's of the tank will yield the most accurate results. Just keep your methods consistent, if you top off, top off every time. if you let the pump click and stop, do that every time. It is true that the more time you spend driving the 5, the better you know how many gallons you have at a certain spot on the fuel gauge. We know that the 5 has a 15.9 gallon tank, and that the gas light comes on when you have around 3 gallons left, so calculating from there makes it a little easier.
I only wished we could have 2 differents 'fuel maps' or ways the engine is managed at a flick of a switch. So that we could keep the 'ZOOM ZOOM', or be ECO when we feel like it.
I believe that in cases like yours, with mileages computing to be around 17 mpg, and those computing to be around 32+ mpg, are invalid results and are worthy of being thrown out...