Bottom Line
Improving Fuel Economy part 2
Spec'ing and maintaining

Since last issue's article on driving to save fuel, the price of diesel has climbed to new heights. With drivers paying about $1.70 a gallon (more in some areas), the difference between 6.0 mpg and 6.5 mpg easily exceeds $2,000 per 100,000 mile year. That could mean the difference between survival and struggle, or perhaps failure.

Fuel economy is a direct function of the energy consumed by your rig. That energy can be wasted by excessive speed, rapid acceleration or flexing under-inflated tires. It can be absorbed by the brakes and dissipated into the air as heat. And it can be wasted by having to overcome improper settings or the effects of inefficient equipment. In other words, you need to spec properly and maintain your rig for better fuel economy.

Aerodynamics make a huge difference

In October, we looked at how speed affects aerodynamic drag. Now, let's look at differences in design. While most of us, myself included, love the classic look of a long nose rig with a big, flat bumper, the newer aerodynamic designs have it all over the classics when it comes to fuel economy. The difference between a T-2000 and a W900L, or a Pete 387 and a 379, all with identical engines, transmissions, axles and tires, can be more than one mile per gallon. That's $4,000 per year or more at today's prices.

Aerodynamics is a function of frontal area (which is similar for most trucks regardless of design), the square of the speed the truck is going (which is independent of design) and the truck's coefficient of drag. The coefficient of drag varies from truck to truck. It is a function of how fast and how often air must be displaced to make room for the truck. A broad, flat front forces air violently to the side, making it turn 90 degrees. A pointed front cuts into the air and makes if flow at a less severe angle. Moving air to the side takes energy. In some trucks, the air must first move over the fenders, then around the air cleaners, and again around the cab, creating three movements instead of just one or two. Cab extenders and matching roof-mounted air deflectors keep air flowing smoothly. They can improve fuel economy by as much as 5 percent by reducing turbulence behind the cab and in front of the trailer. One of the most aerodynamically efficient trucks ever made was the Peterbilt 372 cabover. It had a sloped, angled windshield, angled front, and smooth sides all the way to the rear. Air moved efficiently, and it only moved once. The newest aerodynamic conventional tractors are almost as good.

Tires play a role

Last issue we looked at tire inflation. Tire design also plays an important part in fuel economy. If you need the superior traction of lug tires, by all means get them. But for the on-highway driving that most of us do, ribbed tires provide more than enough traction while they resist the flexing that heats and destroys tires.

To demonstrate the point, take a paper clip and bend it open. Then flex it back and forth with your hands. It gets hot in the center where it flexes. Eventually, it will break. Do this with several paper clips. Flex one a great deal, a half-inch in each direction. Flex another only a quarter-inch. The more a paper clip flexes, the hotter it gets. The same is true of the frequency of flex. More equals hotter. This is exactly what happens to the steel cords inside your tires. Rib tires flex less than those with blockier treads. And low profile (low aspect ratio) tires resist flexing better than tall rubber.

Air is a structure part of a tire. The higher the air pressure, the more rigid the tire and the more it resists flexing. If you plan to drive more than 65, you may want tall tires to take the speeds and loads better. Driving more than 70, you should increase tire pressure 5 to 10 psi for every 10 mph over.

Where does the energy to do all this flexing and heating come from? There's only one place it could come from - the diesel fuel your engine burns. Tire manufacturers tell us that 15 percent under-inflation can reduce fuel mileage by 2.5 percent. If you're getting 6.5 mpg, that drop to 6.34 mpg can cost you 294 gallons per 100,000 miles, or about $500. Tires are one component of what is called rolling resistance. The term encompasses anything that absorbs energy as the vehicle moves, other than engine heat loss and aerodynamics.

Fuel saving devices

Beware of devices that promise huge increases in fuel economy or horsepower, but can't back-up their claims. Time-tested and industry-accepted items like the Fuel Preporator usually operate on sound principles of physics. Removing air and maintaining temperature help fuel burn more consistently.

But there are still things like cow magnets that strap onto fuel lines, or magic "ceramic infusers" with "rare and precious metals." They are often made "according to a secret forumla smuggled out of the Russian space progra," but their claims are backed by absolutely no verifiable testing according to industry accepted test procedures. The $400 to $2,000 that these devices cost will buy a heck of a lot of diesel.

Another factor in rolling resistance is wheel and tire alignment. Many operators merely check steer tire alignment, but as we reported in recent issues, The Maintenance Council (TMC) recommends "Total Vehicle Alignment." If all wheels are pointed in the same direction, the vehicle will move smoothly with minimal rolling resistance. But axles do not always stay in line. Bolts loosen, shackles shift. Tie rods bend, or if adjustable, they loosen. Vibration and normal wear and tear take a toll. A misalignment of just one-tenth of a degree can drag a tire sideways about 100 miles in 100,000 miles of travel. That scrubs rubber from the tread. Besides setting-up uneven wear patterns that prematurely wears tires out, the scrub requires energy, and again, it comes from fuel. With 3/8" of toe-in and 1" misalignment of drive and trailer tandems, fuel economy will drop about 2.5 percent more, another $500 or more.

Rolling resistance also includes driveline losses due to friction. Synthetic lubricants in the transmission and drive axles have been proven in SAE/TMC approved tests to improve fuel economy by about 1 percent.

Engine power: the right stuff

Spec'ing sufficient engine power can help reduce fuel expenses. For years, the theory was that the greater the engine's power, the poorer the fuel economy will be. That was absolutely true of mechanical engines that were optimized for peak power, and were inefficient at lower output where most driving was done. Today's electronic engines are completely different. The electronics adjust fuel flow for all possible conditions, and will not inject more fuel than the engine can use efficiently. An advantage of large, higher horsepower engines with torque ratings of 1,750 lbs. ft. and higher, is that there is less need to shift down when climbing hills. When you stay in higher gears longer, you burn less fuel. When members Bob and Suzanne Stempinski swapped their 500 hp Detroit for a new, 14 liter 550 with 1,850 lbs. ft. instead if 1,650, their fuel mileage actually increased several tenths of a mile per gallon.

Spec'ing your new truck for fuel economy

When ordering a new truck, have your dealer run a computer simulation for your truck's configuration. In the past, if drivers ordered the wrong gear ratio or an engine without enough torque, by the time the owner found out, it was too late. Now, you can make projections ahead of time. Pay particular attention to start-ability and grade-ability in each gear. You want the highest gear ratio (lowest numerical ratio) that will yield acceptable performance. You can make changes on the computer to refine your choices before placing your order.

A few years ago, I had a driver call to tell me about the new truck he was getting. It had everything a driver could want, including the biggest engine available at that time. My friend, who shall remain nameless, heard about the recommendation from TMC to "Gear fast, run slow" for maximum fuel economy. He had an 18 double-over transmission coupled to a 2.90 rear end. On paper, the truck was good for well over 100 mph, but it was geared so high, it wouldn't have been able to climb a 1-percent grade without dropping down to about 12th or 13th. Luckily, we caught the spec before the truck was ordered. By changing only the rear-end ratio, the truck was delivered as good as it can get.

The idea of gearing fast is fine, as long as you drive slowly and use the power to climb hills when you need to. Every dealership has a computer program and should be able to run all the options for you. If you're in the market for a used truck, I suggest you ask the dealer to run a simulation for the truck's specs. Then you'll be able to compare trucks, and you won't have any unpleasant surprises.

Accessories can help you conserve fuel

Idling is often important, if not downright necessary, for driver comfort. But idling can be avoided by using alternative devices. Window screens and a roof vent can help keep cool in the summer without the need for air conditioning. A fuel-fired heater can keep you warm burning one gallon a night, compared to eight gallons to idle an engine. Seven gallons saved times 200 nights a year, at $1.70 per, equals $2,380 per year. That represents better than a one-year payback for the heaters, and that much fuel saved per year. I spoke with one Canadian driver who hauled beef from Calgary to Montreal. He had an Espar Combo Kit. He would brag about shutting off his engine, getting undressed and going to bed...with the outside temperature at -40 degrees.

Spec'ing for fuel economy involves the shape of the truck, and drive train specifications. It entails proper selection of tires and a wise choice of accessories. Maintaining for fuel economy involves keeping the fuel system in top shape with detergent additives and conditioned fuel, keeping all wheels, including drive and trailer tandems, aligned and tires properly inflated. But even the best possible maintenance of the best possible truck won't begin to equal the effect that your driving technique has on fuel economy.

Paul Abelson is Land Line's technical editor and freelances from his office in Lisle, IL.

Aug/Sept Digital Edition