Unless I missed it, no one has asked in what driving circumstances you're getting 13.8 MPG. Is this stop and go city? Highway? A mix of both? How you're driving makes a big difference.
For our lightly modified Xterras (some armor, slightly larger / heavier tires) around town, short trips and lots of stopping and going, 12-13 is common.
On highway trips going 55-65 MPH, that number rises to around 18 and can go as high as 20.
Above 65 MPH, like long freeway trips going 75-80 MPH, and it drops back down to 16 or so.
I agree your aftermarket bumper hurts mileage; it will have a greater impact at higher speeds though because of (lack of) aerodynamics.