I let the DTE tell me I had 2 miles to go and filled it up with 21 gallons. Maybe the fuel pump in the tank takes up ~3 gallons of gas space.
I usually fill up within a few miles of the the DTE telling me I have 40 miles to go and it has never been even close to the same amount of gas. Sometimes it takes 17 gallons and sometimes it takes 20 gallons even though it all says 40 more miles to empty.
It probably calculates DTE on the fuel mileage from the EVIC but mine is pretty consistent at 11.5 MPG for the past few weeks and it is still all over the place. My Fusion is dead on with the readings, when it says I have 20 miles left (which is my average gas mileage on the dash) I fill up and it is right on 16.5 gallons (the tank is 17.5).
Oh well, I know to fill up earlier than later with the Jeep because of the sloppy calculations it makes.