Cant speak for everyone, but in my experience a totally stock 7.3 wont ever exceed 14/16mpg/hwy empty, depending on terrain, and thats only if you drive like a granny, and keep the rpms at or below 2000 on the highway. change the intake, delete the muffler or go 4"exhaust, yer looking at 16s consistently if, as stated, you granny it. Add a chip, depending on which one you choose, and yes, its fully possible to hit 18mpg or even better, IF YOU GRANNY IT!! As soon as you start rabbiting off of stoplights/signs, or doing better than 2000rpms on the highway, start saying goodbye to fuel economy. The temptation to drive these trucks like sportscars is monumental. Good luck.