I bought my F250 2wd crew cab 7.3 new, and on the open highway it would get about 18 mpg.
I added a 80 hp econo mode chip from DP Tuner, and it went to 20 mpg on the open highway (normal speeds.) I've had not one problem in 10 years' usage.
The problem is keeping your foot out of the throttle, and running it under 70 mph. Most owners don't have the self control to baby the pedal.
My truck would just chirp a tire from a stop. Now, I can burn the tires from a stop. It's almost dangerous to drive in the rain or snow, as it's got so much power from a stop. And 60-80 mph comes up in 4 seconds flat--strong passing gear.
See DieselStop.com if you have any questions about this fine engine/transmission combination.