A drop in mileage can be due to a ton of things such as not changing fluids (diffs, trans, transfer case) causin increased drag. Bearing going on the wheels causing increased rolling resistance. Not changing spark plugs or air filters with oem or better replacements. Even different tire tread can affect this. Your issue of seeing decreased fuel economy vey well may be due to the decreased energy content per gallon of fuel due to ethanol. There is no truth to the idea that engines will have decreased fuel Economy under normal use unless it was neglected.
2011 WK2: Limited, 4x4, Hemi, 730n, Class IV Tow, Brilliant Black. 30% Ceramic Tint, Lockpick, LED DRL, LED License Plate, LED Backup, LED Liftgate (all), Axle/Transfer Case OEM add-on skid plates, Sharkfin Antenna.:thumbsup: