I have a 2011 Ecoboost SC 4x4 with a 3.73 rear end. I have noticed lately that my mileage is crappy. I'm talking barely getting 15mpg on the highway. My tires are BFGoodrich 10 ply all terains and slightly taller than stock, but I was still getting around 18-20 mpg on the high way with them on. I've also installed a K&N Air filter (just cleaned it about a week ago), and recently installed a 4" heartthrob exhaust after watching an episode of truck tv (or something like that) showing quick and easy steps to get the most out of your ecoboost. I thought the exhaust would help my fuel mileage, not hurt it, but after installing it I've only noticed about a 4mpg drop in my truck. Does anybody have any idea what is going on? Thanks. BTW, I also use AMSoil full synthetic motor oil.