It's simple: Diesel owners pay less at the pump - not per gallon, per horsepower. Diesel fuel has more energy in each gallon than gasoline; and when you increase the performance of your truck, you are maximizing its energy output. Meanwhile, folks running gas engines will see a drop in fuel economy when their engine is modified for higher output.
Do you feel superior to the gas-engine masses when you climb into your diesel truck? You should. Each gallon of diesel is packing more heat energy than the same amount of gasoline. | 
By measuring the flow of fuel and the horsepower created at a specific rpm, your brake specific fuel consumption efficiency rating can be calculated. Ask your dyno shop if they can measure your truck's BSFC. |
Modern turbodiesel engines, such as this Duramax, are designed to produce incredible amounts of power and cleaner emissions, but that doesn't mean they are running at maximum efficiency. Most power upgrades can also result in better fuel mileage. | |
Unlike gas-sucking big-blocks, today's modern diesel engine is capable of producing more power than the stock configuration without losing fuel economy. In fact, modifying a factory engine will often yield better mileage. This can be verified with a dynamometer using a calculation called "brake specific fuel consumption." To obtain a BSFC efficiency rating, a fuel flow meter is used and the pounds of fuel used per hour are divided by the brake horsepower (see sidebar).
To use a real-life example, imagine your 7,000-pound truck traveling down the highway at 70 mph and getting 14 mpg. This means you are burning about five gallons of fuel per hour. At about seven pounds per gallon, you are burning approximately 35 pounds of diesel each hour to keep that 70-mph pace. When running at peak efficiency, a diesel engine uses about 1/3 of a pound of fuel for every horsepower produced. In this example, the truck is not running at peak efficiency, so let's say about a 1/2 pound of diesel is being used to travel at 70 mph. That means 70 hp is needed to maintain the cruising speed, and the BSFC efficiency rating is 0.50.
Enhanced flow fuel injectors like this Bosch unit improve the dispersion of diesel inside the engine cylinders. This increases the quality and efficiency of the combustion, which equals more power and better fuel mileage. | 
By optimizing the timing of fuel delivery, an engine programmer and fuel module can unleash ponies that are trapped by the stock engine configuration. |
"Unlike gas-sucking big-blocks, today's modern diesel engine is capable of producing more power without losing fuel economy."
Since there is no air throttle in a diesel engine, the amount of fuel that can be delivered to the cylinders is the limiting factor in how much power is produced. So, it's easy to gain horsepower when you add a fuel module, power programmer, enhanced flow injectors, etc. Now the engine will be more efficient at making the 70 hp needed for cruising at 70 mph. After the upgrades, imagine the engine operating at full efficiency and using just 1/3 a pound of diesel (per horsepower, per hour) to make the 70 hp and you have a BSFC rating of 0.33. Multiplied by 70, that comes to 23.1 pounds of diesel (3.3 gallons) consumed in the 70 miles covered during one hour. Now that 14 mpg has jumped to an amazing 21 mpg. Plus, you have plenty of extra power on tap and some extra dough in your pocket to spend on the back tires you will be smoking.
Brake Specific Fuel ConsumptionTo calculate your BSFC rating, you divide fuel flow rating at a specific rpm and divide it by the horsepower output at the same rpmBSFC = fuel pounds per hour / brake horsepower