If you can see verifiable decreases in the energy needed to move the vehicle, say via a coast down test or similar, but no significant change in mileage, you need to alter your driving habits/gearing to allow you to take advantage of the efficiency improvements. You have essentially two things to look at, the energy needed to move the vehicle, and the efficiency of the engine. Ironically enough, the more energy it takes to move a vehicle, the more load is put on an engine and the more efficient it operates. Similarly, if you start making the vehicle more efficient, for instance through a sloped bed cover, lowering, tires at 50psi, etc... (see
Big Dave's truck for examples), the engine will tend to operate less efficiently. In order to maximize efficiency you need to increase the vehicle efficiency by making it roll/move through the air easier, and insure that the engine is also operating as efficiently as possible through gearing or maybe driver habits.