Diesel vehicles work in a different fashion than normal gasoline fed internal combustion vehicles. They subsist from compression of the fuel gas mixture without the use of spark plugs to ignite the mixture. By compressing diesel gas to a high extreme it causes the fuel to combust from the high pressures without the help of any spark plugs to ignite the mixture. The same general rules apply for increasing fuel economy of diesel vehicles and trucks as do for regular gasoline vehicles such as decreasing highway speeds, not letting the vehicle idle excessively, etc.
One method of reducing fuel consumption in diesels and improving your gas mileage is to purchase and install a power programmer that is geared towards fuel economy or has an improve fuel economy option setting. A power programmer can also be programmed to improve gas mileage by the manufacturer if they choose to sell such a model. This is done typically with diesels by programming the transmission to decrease its shift points in automatic transmissions. By decreasing the shift rpm in each gear your diesel will waste less fuel and be more efficient because the higher the rpm the engine is operating at the higher the fuel consumption in general as a larger amount of air is being vacuumed into the motor per minute. This means a larger amount of fuel is also being combusted in the chamber. And as we know the more fuel that is burned per unit time, the larger the fuel waste (because 100% of the fuel is not converted within the chemical reaction, so the more of it you burn the more money you waste).
Generally normal power programmers for gasoline engines decrease fuel economy because they lower the air fuel ratio to burn more fuel and create more power in the process. So if you need your diesel to increase its fuel economy then make sure you get a power programmer specifically for raising fuel economy. Do your research before you purchase it. Certain power programmer companies like Bully Dog even have integrated software into their tuners that shows the driver how to alter their habits while driving to improve fuel economy.