There are a number of factors that affect real world fuel economy: weather conditions, driving conditions, driving habits, etc. Also, most trucks and cars are equipped with Vehicle Control Modules. These modules are constantly taking in information from a variety of sources. The computers need to be reset over a period of hundreds of miles and even a couple of tanks of fuel.
You will see immediate emissions reductions and may feel a slight power increase due to the DieselMaxx/GasMaxx providing a better grade of fuel to the engine.