A church friend with the 3.3L Supercharged Frontier said 15MPG is the best he ever got in 150k miles of ownership, and usually it stayed around 13MPG. Down to single digits when offroading... We called it banana, as it was yellow, crew cab, with the longest bed option.
By way of comparison, I get 25-26MPG in my 5-speed manual Frontier with the 2.4L I4 running on E15 gas (which is pretty much all I use in it).