I run stock size P275/65r18 General Grabber at/2 on my Ecoboost. I have lost 2-3mpg's since changing out the stock Goodyears. I went with the General's because they were one of the lightest AT's I could find @44lbs. Weight and rolling resistance seam to hinder mileage on the Ecoboost powered trucks more than the 5.0.
2012 F150 SCAB XLT/4x4/EB/3.55 ELD/Livernois 93v10 & Off-road down pipe