There are many things that determine what kind of fuel mileage your diesel powered truck will get, but one of the most influential and easiest to control is how you drive. As an added bonus, it is also the cheapest and easiest to fix.
Modern diesel truck engines make such huge amounts of torque because of two main factors:
1) They are turbocharged
2) They have modern electronically controlled fuel delivery systems with high pressure injectors.
This leads to great power for towing and climbing, but it also means that when you put your foot into it, your mileage drops, and fast. Every time you push the accelerator pedal toward the floor, it causes the injectors to put more fuel into the engine. The diesel fuel is ignited, causing increased exhaust flow, and the energy stored in that exhaust makes the turbocharger spin ever faster. The turbo literally forces more air into the engine, where it mixes with the fuel and burns. The more diesel fuel you call for with your right foot, the more air is needed.
Without turbocharging the engine reaches it's limit of how much fuel it can use fairly quickly. With the turbo forcing additional into the engine however, the more fuel it can and does use. You get the benefit of dramatically increased power and torque, but mileage suffers along the way.
The other thing that severely affects fuel mileage is how fast you drive. This is especially true in large pickup trucks, which are about as aerodynamic as a barn, door and all. Moving air behaves like a fluid, and air resistance increases geometrically, rather than mathematically, so that driving twice as fast requires using 4 times more power to overcome air resistance.
Since pickups are not very aerodynamic and have a large surface area to push through the air, your speed effects fuel economy much more than it would if you were driving a small, aerodynamic car. So trim back your velocity on the freeway and you can gain up to a few MPG, depending on your vehicle.
Two main points to increase fuel mileage in your truck:
1) You're driving a large vehicle with poor aerodynamics; slow down.
2) The more you put your foot into it, the more boost pressure you'll make and the more fuel you'll be able to burn.
