I love and hate these "gadgets" new cars have. I love all the information, but hate it because I overthink it. Many of these readouts arent overly accurate. and like DGXR states, a 5% difference is almost too small to worry about. It could be a variety of items such as what has been listed here. That readout is just an estimate from the other items the computer has to guess the MPG, kind of like the miles to empty. Every so ofthen, do the actual math and that will give you a better indication if you are having a drop in MPG. New cars are sensitive, if the car was having an issue, most likely a check engine light would be on.