The sending unit in the tank might either be defective or something is restricting the float from going all the way down. I have no idea how the trip computer calculates MPG. I have run larger injectors in a couple of different Ford applications with the trip odometer, so I don't believe that is an issue in the calculation. I have an app that I use to keep track of actual fuel consumption in all my vehicles. The trip computer isn't all that accurate with a bias to make you think your mileage is better than it is. At least that's been my experience with 4 different Ford vehicles.