Hello, I just purchased a new 2017 Dodge Durango R/T RWD. I am still on the first tank of gas (100 miles), but I notice the MPG is averaging 8-9 MPG both city and highway. How is this possible, the vehicle says its rated at 14-21 (17 average). Any suggestions? Is it that I need to break the vehicle in first? The eco mode is on, sport mode is off.