That all being said I was excited to go on a short road trip with my wife this past weekend both to get away from the kids and to see what the hwy mpg would be. On cruise control at 75 mph (120km/h) with 2 stops for construction I got 16.4 MPG (14.3L/100km)!!!!! I get the same if not better mpg with my brick of a '01 Xterra!
You may be right, however the software I use doesn't rely on a FULL tank. I enter the amount purchased and then the distance travelled and i get my mpg since last purchase/recording. I could only put $10 in and it would still give an accurate mpg.
I'm not sure why the 5 was so thirsty on that trip but plan on monitoring things closely before spending $$$ to get it looked at. I may have to find a reason for another road trip! ;-)
But I kept adding until the fuel was right up to the neck of the fill tube.
I've never had an issue topping off any fuel tank. The charcoal canister is installed below the top of the tank in the 5, so I'm sure the PCM doesn't purge unless it knows it's safe to.
So, when I have to stop for gas and the temp gauge has barely moved, I leave it running.
I say this to tell you that when I 'top off', even when the engine is idling, the fuel drains down rather rapidly from the filler neck.
Simple. The fuel expansion is nill, it's the vapor where the concern is. The gas cap is a pressure release, but I doubt there's much problem here because...
I do "No-no" #2 almost every time I fill up - I leave my engine running. When I stop for fuel, it's always after work, the gas station I use is 1 mile away. I'm more concerned about warming the engine up before heading on my way home. So, when I have to stop for gas and the temp gauge has barely moved, I leave it running.
I say this to tell you that when I 'top off', even when the engine is idling, the fuel drains down rather rapidly from the filler neck. It travels down out of sight below the splash valve within seconds. Another thing, the rate of evaporation is relative to exposed surface area of the fuel and the volume that it can expand into. When the tank is full and travels up the filler neck, there's very little exposed fuel surface area, so the vapor is minimal.