This is the truth written above,

I bought a scanguage for shits and giggles and its fun to watch your instant MPG change based on load, speed etc. On the scanguage the difference between 55mph and 70mph is about 3.5 mpg or 16.5 - 20mpg. After calibrating the scanguage over a few tanks its usually spot on or 0.1 gallons off each time I fill up. Get up to about 75-80 mph and your dipping below 14-15 mpg. This is with 33's, shrock, winch, yakima rack, BL etc so a stock X should be close or better if not its all about the driver sorry....

edit to add the Xterra is a pig around town, city driving only I can get under 14mpg