Just got a new f150 4x4 with ecoboost engine. Drove home from Phoenix (100 miles or so) and loved the ride, etc. but not the gas mileage. Was promised 21mpg on the highway but only got 16. I did drive 65 to 75mph most of the way. Is my bad mileage because of that speed; need a break in period, or what? Anyone know what's up here? thanks.