Best way to do mpg...
Fill tank till the pump clicks off (yeah, I know, costs a fortune...)
Drive and use at least half. The longer you drive, the more accurate the reading.
Refill tank till it clicks off, as before.
Read gallons on pump. Jot it down.
That's your gallons consumed...divide that by the number of miles between your fuel stops, and you'll know.
Highest you should expect with these trucks is around 15-16 mpg, and that's rare.