I have a flowmaster super 40 and K&N FIPK intake on mine, drive about 70% highway, I'm getting around 16mpg on average for each tank. I have 17,000 miles on it and ikeep up on my oil changes and the such. City driving I get about 12mpg, if I drive all highway I get about 22mpg.

 

I have a 4.7 2x4 and I am getting between 14.6 and 17.4 depending on how much highway and "city" driving I do.

 

My son has only checked his once, it was combination driving and he has a rather heavy foot. It got 18.2 mpg. There are no mods. It is a 4.7L auto. Not sure what gearing.