Same experience for me with 4x4 Lariat but after 900 miles and third tank of 87 octane the mileage started to rise. Currently 21-22. I have now started to play with eco vs standard. Currently getting 24 on this tank. I will admit to being very smooth on accelerate and braking with minimal idling...
I too tried everything and finally the dealer created a service ticket. They took part of front grill off to raise the whole closing latch. Works fine now. Clearly a quality control issue.
