Agree redwngr, another form of, "you gotta pay if you want to play"
With the poor aerodynamic form of big pickups, increased speed kills fuel economy due to air drag so unless you always plan your travels to run with a big tail wind it will cost you. Every time you double your speed on a flat road it takes approximately 8X more horsepower. Normally doubling the rate at which work is done would require only twice the horsepower but because the air resistance follows approximately a square law response you end up with 4 times the air resistance at twice the speed coupled with the need to increase horsepower to do the same work at a faster rate you end up wit the basic 8X equation.
Or put simply, it if takes 10 horsepower to travel 40 MPH your horsepower demand goes up to around 80 horsepower at 80 MPH and you will burn more fuel to generate this power.
So why don't you get 8X the fuel economy at half the speed? The 8X approximation is a pretty close rule of thumb at high speed but at lower speeds you are also overcoming additional drivetrain losses, rolling resistance, any road grade, etc. as you increase speed and the powertrain is designed and geared to produce best efficiency under typical operating conditions. So even at lower speed you aren't going to be challenging a prius in the fuel economy race but at 52-55 MPH you should get in the low 20 MPG range which might be important if you are very low on fuel many miles from the nearest station. On the other hand if you remove the speed governor from the programming and get the truck into the 110 MPH range then you probably will be in the 3-4 MPG range or about what you would get running the truck fully loaded up the Ike with the engine RPM at peak HP.