I got my 2006 Limited 3.7 this last summer. It was getting 17MPG.
About a month or so later I gently seafoamed by adding to the oil, gas and a little through the suction hose as recommended. Never stalled out. Afterwards I cleaned the intake and changed out the plugs (which did not foul from the seafoam)

I drove around 200-250 miles max with seafoam working in the oil before changing it to full synthetic amsoil. Needless to say it came out very black and nasty.

After I changed the oil, the gas mileage went down to an average of 13-14 MPG tops and stayed this way regardless of city or highway driving.
I changed to part synthetic Valvoline 5W30, the Amsoil was absolutely filthy. Now i get a max 15.8 highway and still 13 city. The engine itself seems to run fine and I've done a tank of Heet to remove any moisture and a couple in-gas-tank injection cleaners.

is this MPG abnormal? It should be known I have nicer and New Yokohama street (mud + snow) tires on there, so nothing offroady that may affect mileage.

Oil is not leaking and all my plugs look fine. No weird smoke coming from anywhere, no burning smells. Recently changed out the coolant, put on a new water pump and belt.

Any ideas? Or...? I see most people seem to think 17MPG is the normal with non-agressive tires.

My mileage went down from 17Hwy/15City to 13 hwy and 13.5 city when I put in amsoil full synthetic and also an Amsoil brand air filter. Changed the oil to half synthetic and got up to 15 Hwy/13.5 city. Put in a Fram air filter as a test to replace the Amosoil filter and my Highway mileage today went to 20MPG.