[brucepick]

Get a ScanGauge or UltraGauge. Either will plug into the truck's OBDII connector just like a code reader. The gauge will give instantaneous mpg, actually updated every 1-2 seconds. Also provides trip and tank mpg data, as well as useful information such as engine temperature, intake air temp, % load, etc.

That instant feedback is invaluable in refining your driving technique. I learned that my car - which averaged over 50 mpg last summer - gets only about 10 mpg when I stomp on it in 3rd gear. You don't need to be a rocket scientist to predict reduced mpg in third with foot down hard, but there's nothing quite like seeing the numbers in front of you, real time.

Good luck with it!

And you can use the same gauge in the fuel sipper car suggested above by jamesqf.