My son Eric has built his XJ up to where it's not fun to drive to Moab and St George and decided to get a tow rig. He bought a 2013 EcoBoost yesterday with the 3.5. Nice truck but needs a few things and some TLC. Computer says that he's is only getting 15.2 mpg on the freeway. Can that be true? Is there a way to improve that. Not sure if that's adjusted for tire size or if it needs an air filter