When it comes to choosing between a large- or small-displacement engine, sometimes the correct choice is not obvious. "The small vs. large displacement debate is really not that simple," says McKenna. "Several years ago, an engine manufacturer supplied two displacements from one block. To everyone's surprise, the larger displacement was somewhat better on fuel economy. This was because the horsepower density (horsepower per liter) was more balanced.

"Think of your engine as an air pump," he continues. "At which size can you most efficiently get the air and fuel in and the exhaust out efficiently? At Mack, we like to see something around 32 hp per liter +/- 3 hp in our MP7, and 34 hp per liter +/- 2 hp with our MP8 and MP10."

Always consider the environment in which you plan to operate the truck. "When spec'ing the vehicle, typical operating conditions should be identified (terrain, grades, road speed limits), so the proper gear ratios are selected to ensure adequate startability and operation without sacrificing fuel economy," says Gervais. "In addition, transmission features such as overdrive, automation and progressive shift programming can have a positive affect on fuel economy."

The power train must be designed to work together. "Gearing can have a negative impact on fuel economy by operating the engine outside of its sweet spot on the fueling tables," says Nycz. "Numerically higher axle ratios do improve startability and gradability, but at the cost of fuel economy.

"All engine manufacturers have minimum guidelines for these values to maximize performance and minimize fuel consumption," he continues. "The gearing requirements of a vocational application are usually numerically higher to allow satisfactory operation on off-road and soft surfaces."

Practice diligent maintenance
"With diesel currently above $4/gal., a comprehensive preventive maintenance program will pay for itself in fuel savings alone," says McKenna.

Gervais agrees, noting, "Maintaining engine components such as air and fuel filters keeps your engine running at peak performance, resulting in more efficient combustion and better fuel economy."

All maintenance factors affect some aspect of performance and fuel economy. "But the most critical maintenance issue that directly impacts fuel economy is air filter maintenance," says Powers. "The engine needs an unrestricted supply of clean air to operate efficiently."

Tire maintenance also ranks near the top. "Properly inflated tires mean less work will be required from the engine to move the vehicle, with the end result being improved fuel economy," says Gervais.

"Improperly inflated tires can drastically reduce fuel efficiency, as well as diminish the life expectancy of the tires," Powers adds. "For example, tires that are 10 psi low can decrease fuel economy by 0.5%."

Pay close attention to axle alignment, as well. "Axle misalignment will adversely affect fuel economy as much as 2.5%, depending upon the number of axles misaligned," says Nycz.

"Proper axle alignment reduces tire scrub, resulting in less resistance and improved fuel economy," says Gervais. "This holds true for tractor axles and trailer axles."

The importance of axle alignment to fuel economy really depends on how much time the truck spends on the road. "The greater the amount of time the vehicle spends at road speed, the greater the savings or cost," says McKenna.

Keep track
As with any performance criteria, tracking the impact of any changes is important if you expect results.

"Monitor the results of any changes to your operations ? such as road speed limits, idle shutdowns, PTO settings, tire type and tire size ? as it is possible to make fuel performance worse," says McKenna. "Make one change at a time so you can quantify the improvement for each change."