I know this is a much talked about topic but I'm just trying to find an answer. About a year ago my e99 f350 was getting 17mpg average on the interstate doing a constant 70mph. The truck died one day and turns out it needed a new pcm so the dealer put one in. Anyway, since that day my mpg have never gone above 14mpg town or hwy. I'm not mad, I just know I was getting 17 and want to figure out why the mileage dropped. I've put a 6637 intake on, ebp tube is cleaned and sensor is new, 4in Turbo back exhaust. 235/85/16 tires. New intercooler boots all around, bellowed up pipes. My brakes aren't stuck I just check 5 days ago. I thought it might be a faulty pcm so I put a chip in from dp tuner with custom tunes. That didn't fix it. I've scanned it with an AE scanner and everything checks out good. I have 3.73 gears at the back. It's 4x4. No my hubs aren't locked in. Oil is good and fuel fter is clean. Any ideas? I've tried any mpg mod I can think of and nothing has worked. I just don't believe I can get 17mpg then instantly drop to 14 and never get more.