Maybe we can simply return to gathering data and leave the "is 3-4 gallons a normal reserve or not" question behind for now.
Those of us experiencing the issue first-hand recognize that the car is requiring fuel earlier than we expect if it indeed is filled with 16+ gallons of fuel. We want to gather as much information as possible regarding our experiences and the experiences of others so that we can work with our local dealers to determine what to do about it.
I am an avionics troubleshooter as part of my work and the first step to figuring out any resolution of a technical issue is to gather data and then organize it into relevant and non-relevant information.
We have accomplished an initial round of this with regards to which cars seem to be affected. It appears that none of the Hybrids nor AWD cars are seeing short fill-ups. Those owners have thus far reported that their cars take the amount of fuel they expect. Based on this, it would suggest that the issue is not related to the capless system since that is something all of the cars share. The one difference is the actual tank installed in the non-Hybrid/non_AWD cars...the 16.5 gallon tank.
One note: Fuel tanks are not just metal bags...they are complex structures with supports and reinforcements to make them crashworthy and the internals are designed with various baffles so that the fuel doesn't slosh around and is consistently available to the fuel pick-up or pump even when the level is very low. Even if it is rather rare, it is not far-fetched to think that there could be build errors that might prevent normal operation.
At least 2 people from the other forum have had tanks replaced with positive results. This also backs up the idea that the tanks may have a defect of some kind. One of them even got that specific feedback from Ford engineering if we are to believe their story.
The issue now is that getting a dealer to do a tank replacement under warranty is not a normal activity for a brand new car and is likely something the dealers will resist. I would recommend that any of us who are feeling like we have the issue begin discussions over the phone with your dealer as soon as possible to ask them to contact Ford on the issue to see if there are any TSBs or internal notices regarding the Fusion tanks. (For the record, that is exactly what has happened for the headliner issue...Ford has put out a notice that they are investigating the issue...but it is much easier because it is something the dealer service people can see for themselves. The tank issue cannot be visually verified, so it will require more convincing)
I would also ask that anyone who has spoken with the dealer share their experiences here so we can see how the dealers are treating the issue.
If we approach this systematically, we should be able to gain traction on service actions and get a final answer regarding the real issue.
2013 Ford Fusion Titanium - BLK TI "Eleanor"