peterbilt 387 mpg?

i'll second that

 

there's too many factors that determine fuel mileage in a truck. the 2 most important are Driver input and VGW (vehicle gross weight) after that you have a prrrrrety list of 50+ adding to the equation.
so one can say you can do 5.2mpg, others can say you can do 10mpg.. not all trucks are the same, not all roads, weather changes and temperature. tire inflation, aerodynamics, lubricants, truck engine specs, transmission, differentials, and the list goes on, and on, and on..
if you keep shifting at 1200 to save your fuel, your wearing the engine like a terrier is pulling a husky's sled until something breaks..
my .02

i shift between 15-16

downshift as soon as that clock hits 12

 

we all don't drive the same, all engines and specs on tranny's (transmissions lol) and rears make rpm vary. you have to learn what's the "sweet spot" on the engine your driving (you can do this from the manufacturer or a professional mechanic) this way you know what the operating range of the engine is best for fuel mileage, performance and longetivity.

another tip would be

when you're coming up to a hill, let the momentum take you up, forget the speed as long as you don't fall below 55..
then as soon as it starts coming down the hill, caress the accelerator to give it a lil' juice so she can come up to her cruising speed..

that's just 1/1000 things you can do to improve your fuel mileage.