Maintaining MPG

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Good maintenance always helps ensure a fuel-efficient truck. That’s never changed,” observes Patrick McNamara, national account manager, Volvo Trucks, North America, Greensboro, N.C. “But in an extremely competitive business climate, with high fuel prices, and ’02 engines on their way, it’s more important than ever that the work actually gets done. The return on investment for conscientious maintenance has never been better.”

That said, it pays to remember that there are several often-overlooked maintenance procedures and adjustments that can have a noticeable effect on fuel economy.

Under the hood
For example, engine manufacturers acknowledge that engine accessories can drink up to 18 percent of the fuel used to move a truck down the road. A spinning cooling fan alone uses about 20 horsepower. If the fan clutch is operating improperly, that is, not disengaging the fan when it’s not needed, that 20 horsepower is wasted pushing air, instead of helping to move the truck.

Speaking of pushing air, if a vehicle’s air compressor unloader is out of adjustment, the compressor can spend more time running than necessary, which uses extra energy and wastes fuel. “Air system leakage also will cause the compressor to run more than it normally would,” adds Scott Pearson, general marketing manager, Peterbilt Motors Co., Denton, Texas. “And it doesn’t have to be a major leak.”

While everyone knows that a dirty air filter can deprive an engine of the air it needs for optimum combustion, and can result in less efficient operation, filter manufacturers warn against replacing air filters more often than necessary, because new air filters actually become more efficient (trap more dirt) with use. That’s because a porous cake of dirt builds up on the medium, reducing the size of the filter passages and catching more particles. Changing an air filter before this dirt-pie forms will allow more dirt to pass into an engine, causing faster wear. Also, the more often an air cleaner is changed, the more likely it is that contaminants will sneak in during the operation.

Changing air filters on the basis of restriction-gauge readings, says the Technology & Maintenance Council (TMC), is the most efficient method. TMC also recommends that gauges be periodically removed from air cleaners, and their operation checked with a vacuum pump.

“It used to be that a dirty air filter would cause an overrich mixture and result in black smoke,” notes Tony Cook, chief engineer, powertrain systems, International Truck & Engine Corp., Chicago. “That would tell you it really needed to be changed. But today’s electronic engines will compensate for reduced airflow. They won’t smoke, but performance and fuel economy will suffer. The telltales are gone; you have to rely on restriction readings.”

When changing air filters, Freightliner Corp., Portland, Ore., recommends completely inspecting the air intake piping from the air cleaner to the engine. Look for loose connections, cracks, torn or collapsed hoses, punctures and other damage. Replace damaged components and tighten loose connections. You want to be sure the piping is airtight so all the intake air passes through the air cleaner.

Another good habit, says International’s Cook, is routinely auditing engine electronic control module parameters. “Make sure your programmable limits, like maximum road speed, are still in effect.”

“It’s more important than ever to know how a vehicle is being operated,” agrees Volvo’s McNamara. “Through better use of information technology, maintenance and management can function as an integrated unit, looking at all aspects of a vehicle.”

And drivers can be part of that unit. “If you don’t already have them, consider installing in-cab fuel mileage displays,” suggests Peterbilt’s Pearson. “If a driver can measure his performance, he can improve it.”

Fuels & Lubes
In its “White paper on fuel economy,” Kenworth Truck Co., Kirkland, Wash., reminds fleetmen not to use engine, transmission or axle lubes that are of a higher viscosity than recommended for specific equipment and operating conditions. Higher viscosity lubes are thicker and harder for pumps and gears to move. That consumes more energy than necessary, and wastes a small amount of fuel. (The Kenworth white paper is available at

“You also want to make sure fuel filters are changed at recommended intervals,” maintains International’s Cook. “If they’re restricted, performance and fuel economy are reduced. Again, the old telltales, like a lean engine running hot, are gone.”

Also, “If a damaged fuel filter allows contaminants to reach the injector nozzles, the fuel spray pattern will be disrupted,” says Peterbilt’s Pearson. “That causes poor atomization, less efficient combustion and reduced fuel efficiency.”

And, “When purchasing fuel, don’t forget about quality,” adds Chuck Livering, service engineer, Mack Trucks, Allentown, Pa. “The wrong specs for cetane, viscosity, etc., can hurt performance, fuel economy…and your engine.” Minimum satisfactory specifications for diesel fuel can be found in TMC’s Recommended Practice 304B (

Tire inflation
According to tire manufacturers, tires can contribute, through rolling resistance, as much as 35 percent to level-road fuel consumption. The good part is that, where tire maintenance is concerned, what’s good for fuel economy is also good for tire life.

The relationship between inflation pressure and fuel economy is simple: underinflation equals increased rolling resistance. A soft tire deforms to a greater degree as it rolls against the road. So, more energy is used bending the tire’s belts and flexing its sidewalls. That’s energy that could be put to use racking up miles. Gross underinflation is estimated to cost between 3 percent and 5 percent in terms of fuel economy.

Also, an underinflated tire, with its exaggerated deformation, runs hotter, much as a paper clip gets hot when you bend it back and forth. And because heat degrades a tire’s casing integrity, a softer, less fuel-efficient tire is likely to have a shorter service life. This is true for all tires, new or retreaded, according to the Tire Retread Information Bureau (TRIB), Pacific Grove, Calif. TRIB adds that roughly half the scrap rubber found on highways – the result of casing disintegration from running underinflated – comes from tires that have never been retreaded.

Always check all tires – including trailer tires – when they’re cool, and use a high-quality gauge, says TRIB. Maintain inflation pressures based on vehicle manufacturers’ recommendations, which account for weight, speed and ambient temperature. Only experience in your application will tell you how often to check them.

Toe can be described as the deviation, from parallel, of the longitudinal planes of the two front tires. If the leading edges of the tires are closer together than the trailing edges, the wheels are toed-in. If the leading edges are farther apart than the trailing edges, the wheels are toed-out. Inset: TMC tread cross-section illustration shows feathering, caused by out-of-spec toe. Individual tread ribs wear more on one side than on the other.

Wheel alignment
Wheel alignment is a more complicated issue. When the wheels on a vehicle don’t all agree on which direction to travel, they fight each other, removing a little rubber with every revolution. Fuel economy suffers because it takes extra energy to erase tire tread. In fact, even slight misalignment, say tire manufacturers, can result in 1 to 2 percent fuel economy penalty. “It’s using fuel to waste tires,” observes International’s Cook.

Although the term “front-end alignment” has been around for a long time, opportunities for misalignment, especially on three-axle vehicles, are not limited to the front end. So think of those other axles as added opportunities to save precious fuel.

The three major components of alignment are: caster; camber; and toe. These values can be measured by a variety of means, from string, plum-bobs, protractors and measuring tapes, to sophisticated, computerized, laser-measurement devices offered by a variety of manufacturers.

Your vehicles’ shop manuals will provide proper specs for caster, camber and toe, and will tell your technicians where and how to measure and make adjustments.

Caster is the forward or backward tilt of the kingpin or support arm at the top of a wheel-suspension assembly and is measured in degrees. Forward tilt is called negative caster, and backward is positive. While improper caster has virtually no effect on fuel economy, it can adversely affect vehicle handling and stability.

Camber is the departure, from vertical, of a wheel/tire assembly. Measured in degrees, camber is said to be positive when the top of the wheel tilts outward from a vehicle, and negative when the top of the wheel tilts inward. Again, camber doesn’t really affect fuel economy but, if incorrect, can cause handling and tire-wear problems.

Toe can be described as the deviation, from parallel, of the longitudinal planes of the two front tires. If the leading edges of the tires are closer together than the trailing edges, the wheels are toed-in. If the leading edges are farther apart than the trailing edges, the wheels are toed-out. Toe can be measured and expressed in inches, millimeters or degrees of deviation from straight ahead (zero).

An improper toe setting – usually caused by worn or shock-damaged front-end components – can have a major effect on fuel economy. It also has a significant effect on tire wear, so it’s not difficult to detect an out-of-spec condition.

If both front tires are evenly worn on their outside shoulders, it’s a good bet that there’s excessive toe-in. If the wear is on the inside shoulders, the wheels are likely toed-out. In either case, the tires simply aren’t pointing in the same direction, and each one experiences some road scrub as it argues with the other about where the vehicle’s going to go. This can also cause vehicle wandering.

Toe-related wear, in early, less-severe stages, can show up as feathering, which usually can be felt by hand before it can be seen. Feathering is a condition where the individual tread ribs wear more on one side than on the other.

If the direction of feathering is on opposite sides on the right and left tires (e.g., left on the left and right on the right), a toe problem is a strong possibility. If it’s on the same side (e.g., on the left side of both tires), you’re probably looking at a camber problem.

A small amount of toe-in is built into most steering/suspension systems (TMC recommends a target of 1/16 inches, 1.5 millimeters, or .08 degrees, .06 inches). That’s because, with a vehicle loaded and going down the road, the front wheels have a natural tendency to “run away” from each other. A little toe-in helps keep them parallel while in motion.

Rear-axle wisdom
Rear-axle misalignment on a two-axle vehicle is easy to visualize. When the thrustline, which runs perpendicular to the center of the rear axle, departs from the geometric centerline of the truck, misalignment exists.

The amount of misalignment is expressed in degrees of the angle formed by those two lines. Positive thrust angle means the rear axle is tracking to the right, and will tend to steer the truck to the left. The driver will have to compensate by steering to the right, and the truck will “dog-track” down the road.

On a three-axle truck or tractor, the situation gets a little more complex. Each rear axle may produce its own thrust angle. If both angles are identical in direction and degree, the result is similar to single-drive-axle misalignment. But if the two rear axles’ thrustlines point in different directions, the result is tandem scrub.

Tandem scrub is expressed in degrees of the angle formed by the two axles. This value should be as close to zero as possible. While some rear-axle suspensions are adjustable for axle location and squareness to the frame, most rear-axle misalignment is the result of a bent, broken or loose suspension component, or a worn-out bushing. Keep in mind that even a small scrub angle involves all the tires on the vehicle, and results in a marked decrease in fuel economy and tread life.

For maximum fuel efficiency, drivers, technicians, management and operations must function as a team with the same goal. People make all the difference.

Don’t be a drag
Finally, OEMs agree that aerodynamic devices, such as air shields and fairings, should be kept in good repair. If parts are broken or missing, “you’re not getting the aerodynamic benefit you paid for, and, depending on the type and extent of the damage, you could be creating more drag than if the devices had never been purchased,” says Peterbilt’s Pearson.

And the same can be said of standard body panels, such as fenders and hoods. “Even moderate body damage can disrupt air flow around a vehicle and increase wind resistance,” says Mack’s Livering. “It looks bad, hurts your image and hurts fuel economy.”

Putting it into practice
“There’s no one, big secret to fuel economy,” acknowledges Chad Johnson, vice president, vehicle maintenance, Ruan Transportation Management Systems, Des Moines, Ia. “It’s a number of smaller things that add up.

“The driver is the biggest part of the equation,” he continues. “We routinely download engine ECM data in the shop to make sure our settings, such as maximum speed, are still there. Excess speed reduces mpg and raises maintenance costs.

“After that, it’s basic physics. Any resistance to engine power, or to the truck going forward is going to hurt fuel economy. So we like to make sure engine parasitic loads are no higher than they need to be. And, since a better-running truck is a more fuel-efficient truck, we’re sticklers about staying current on PMs. We also quickly take care of damage to truck bodies and aerodynamic aids, which can increase drag.”

“We’re very careful about tire pressure,” says John DiLuna, manager, fleet maintenance, The Stop & Shop Supermarket Co., Readville, Mass. “We want to make sure it gets checked, so we try to make it easy.” For example, the fleet uses “alligator-type” valve stem caps, which allow tires to be aired without cap removal. But DiLuna advises experimenting to find out which brand(s) hold pressure best. “They’re not all the same,” he says.

DiLuna also insists that every technician have his own air chuck with integral gauge, and that all gauges are regularly checked against a master gauge in each shop.

The result? “During our last survey, 96 percent of all tires in the fleet were properly inflated,” he says.

“We check tractor and trailer air pressure at every fuel-island stop and every PM,” adds Ruan’s Johnson. “You just can’t check too often.”

As far as wheel alignment is concerned, new Ruan vehicles are checked after the first 3,000-5,000 miles, “after the suspension has shaken out and everything’s been retightened,” says Johnson. “After that, tire condition or drivers will tell you if you have a problem.”

Stop & Shop tractors and trailers are regularly checked for even slight misalignment, according to DiLuna. “We use the axle center-to-center method outlined in the Technology & Maintenance Council’s Recommended Practice 642 ( It’s quick, easy, good for your tires, and good for fuel economy.”

In the end, fuel efficiency depends on lots of attention to detail in the shop, “but people are the most important element,” insists Johnson. “Drivers, technicians, management, operations…it all comes down to a buy-in from the entire team.”