Is your engine running good? No exhaust leaks, vacuum leaks, clean fuel filter & air filter, O2 sensors and MAF could be checked too.
I have a 99 F150 5.4 with big tires(stock gears) and a lift with an exhaust leak at the moment(popped a manifold bolt) and I still get 14-12MPG's mixed driving with a heavy foot, if I take her real easy I can bring it up over 15 sometimes. I also have an Edge tuner running level 3 with premium