Stretching The Limits


Here are some practices for stretching your fuel economy and the potential fuel savings:


  • Practice optimal shifting
  • Use automatic transmission
  • Use rib-tread drive-axle tires
  • Stop and start slowly
  • Maintain proper tire pressure
  • Correct misaligned wheels
  • Avoid using grill screens
  • Reduce tractor-trailer gap
  • Add aerodynamic features
  • 6-10%

  • Use cruise control
  • Idle two to three fewer hours a day
  • Use ribs on drive axles, shallow ribs on trailer
  • 10%+

  • Reduce average speed 10 mph
  • Add a full-roof fairing
  • Maintaining a good fuel conservation program is a lot like being on a diet: It requires attention to detail and the drive to keep it going. Sure, it’s easy to grab a burger, fries, soft drink and pie – the equivalent of speeding and excessive idling – even though you know that you should have a salad, a broiled chicken breast, green vegetables and water.

    By mustering the discipline to tweak your driving habits, to stick to certain maintenance practices and possibly to make some equipment changes, you can get 6 to 7 miles per gallon. William Betts, an owner-operator from Steens, Miss., who hauls steel, pays very close attention to his mpg and gets 6.5 to 7 mpg loaded and 9 to 10 mpg empty. “I run the speed limit, do progressive shifting, and spec’ed my 2001 Freightliner Classic to save fuel,” Betts says.

    There’s no small amount of money at stake. If you run 125,000 miles per year, you could cut your annual fuel bill by more than $8,000 if you improve your consumption from 5 miles per gallon to 6.5 mpg (assuming $1.50 per gallon diesel).

    “You have to run the speed limit or under, spec the truck so that it fits the work it has to do and know that it all depends on what you’re hauling,” says Steve Cantrelo of Sulligent, Ala., an owner-operator who hauls specialized loads.

    Following are areas that operators such as Cantrelo and Betts use to achieve significant fuel savings.


    Driving slower is one of the easiest and most effective ways to improve fuel efficiency. According to Kenworth research, every mile per hour over 50 increases fuel consumption by one tenth of a mile per gallon.

    The main reason is aerodynamic drag. Its magnitude depends on the speed, frontal area and external shape of the vehicle. If you have a high average speed, a drop of even 5 mph can make a difference because aerodynamic drag increases exponentially with speed. In other words, you burn some extra fuel if your average speed climbs from 50 mph to 60 mph, but you burn much more fuel as average speed climbs above 60.

    “From 60 to 70 miles per hour we can actually see a drag increase of 50 to 60 percent,” says Bruce Lawyer, senior assistant chief engineer for Peterbilt. The rule of thumb is if you reduce drag by 10 percent, you increase mileage 2 to 3 percent.

    Or consider it in terms of horsepower. Truck maker testing has shown that a loaded, aerodynamic rig, riding on fuel-efficient radials, needs about 213 hp to maintain a steady 65 mph on a level road. The same vehicle requires about 297 hp to maintain 75 mph.

    “I get 6.5 loaded or empty because I don’t run wide open, never over 70, and usually between 65 and 70,” says Sam Long, a Hot Shot Express owner-operator from Morristown, Tenn. “And I pull light loads, from 13,000 to 15,000 pounds.”

    Speeding also accelerates component wear. An engine’s longevity is directly related to the amount of fuel burned over its lifetime. Running a little slower, then, extends the life of your engine and other components.


    Understanding torque band and where you shift is significant. Shifting at the optimal rpm or investing in an automatic transmission can pamper your engine and create a fuel savings of 3 percent to 5 percent. Even simple practices, such as taking slow stops and starts, can boost your mpg.

    You can usually see an improvement if you don’t wind your truck up too much by staying in the sweet bank in the torque curve, at about 1600 rpm. With the new exhaust gas recirculation engines, that sweet spot has gotten much smaller. And letting the rpm run down to 1000 or 1100 before shifting up can save fuel mileage.

    Automatic transmissions can help with fuel savings because they shift up every time you throttle back to cruise, even when a downshift may be needed soon. They also downshift whenever an engine gets well below the torque peak and a driver hits the throttle. Automatic transmissions make the shift at the proper rpm and can save up to 3/10 of a mile per gallon.

    Using cruise control can contribute up to a 6 percent improvement in fuel efficiency, especially where less experienced drivers are concerned. Cruise control helps because it maintains steady pressure, and it can discourage speeding.

    Don’t neglect the obvious waste of unnecessary miles. Driving out of the way looking for your delivery place or for a truck stop with cheaper fuel wastes fuel, so plan ahead as much as possible.


    An idling engine gets zero miles per gallon. Devices, such as auxiliary cab/bunk heaters, coolers and gen-sets can perform functions traditionally dependent on idling, but can cost hundreds, if not thousands of dollars. Depending on how long you plan to keep your truck, how much fuel is saved by avoiding idling, and the climates you drive in, with a few calculations you can figure out if these devices will pay for themselves.

    Some new engines can use as little as a half-gallon an hour idling at 1100 rpm. However, most engines use between 0.8 and 1.2 gallons of diesel an hour when idling, depending on age and efficiency of the engine, idle speed and how many electrical accessories you use.

    Argonne National Laboratories studies show a tractor-trailer parked overnight at truck stops idles about five hours a day, or 1,830 hours per year. Nationwide, trucks traveling more than 500 miles per day idle away about 838 million gallons of fuel.

    Although older engines needed time to warm up, newer engines don’t need as much time. Engine coolant can remain warm for an hour or so when you’re stopped just to eat or shower. Tests conducted by the Ontario Ministry of Transportation investigated whether idling was an effective way to warm up engines in cold weather. By driving the vehicle just after start-up (when the oil pressure is up), the coolant temperature rose from -10 degrees C to its normal operating temperature of 80 degrees C in just 12 minutes. By contrast, it took 30 minutes to raise the coolant temperature to that level while idling.

    A helpful technology on some newer engines is optimized idling. This feature automatically starts and stops the engine to maintain coolant temperature, battery voltage and cab temperature.

    If your engine idles five hours a day, that’s about $2,700 a year if diesel costs $1.50. Bump that up to eight hours during peak summer or winter months, and you could be spending $3,000 or $4,000 a year in fuel without ever turning your wheels.


    Several exterior features can reduce aerodynamic drag, or how smoothly your vehicle slices through the air.

    According to The Maintenance Council, a standard roof-mounted deflector can improve fuel economy up to 6 percent. A full-roof (sides closed above the roof) fairing can contribute up to a whopping 15 percent, with cab extenders contributing another 2 percent. And a front air dam and tractor side skirts can each contribute up to 3 percent.

    The latter serve to shield the high drag of axles, oil pans and other under-cab components. Side skirts also help minimize turbulent near-ground air flow.

    Of course, nothing comes without a price. Add-ons such as front air dams add weight, and when damaged, can lower your aero efficiency until you replace them.

    Peterbilt’s lawyer says one add-on that has such a reputation is the cab extender. “Years ago, they were sheet metal, and they would get bent up when there was a backing situation that had the trailer close to a jackknife, but now they have rubber extenders, and they can really help,” he says. They can still be damaged, but, “If they weren’t worth having on there, some of the most cost-conscious fleets wouldn’t be spec’ing them.”

    You can also save money by avoiding add-ons that hurt aerodynamics. “The big, flat bug screens can affect your fuel economy by as much as 3 percent, and the grill screens affect the air flow through the grill and hood, so you get blockage,” Lawyer says. “We’ve designed it so the air flow goes through the grill and around the hood, where it needs to be.”

    TMC advises that outboard components such as external air cleaners, unshielded exhaust stacks and extra mirrors should be minimized. Even something as small as securing a loose tarp can help your aerodynamics.

    As the space between tractor and trailer widens, air resistance increases. Changing the gap from 45 inches to 25 inches can affect your fuel economy up to 2 percent.

    Buying an aerodynamic truck to start with can make a significant difference in your fuel economy. When Peterbilt introduced the 387, the company said the aerodynamics improved fuel economy 3 percent. Kenworth estimates that the T2000 can increase fuel mileage over a conventionally styled truck by about a half-mile per gallon. The design of the Volvo 780 significantly changed the drag coefficient, which improved the fuel economy 1.5 percent over its predecessor model.


    Maintaining proper tire pressure is a simple way to boost fuel economy.

    “If your tires are underinflated by 10 percent to 15 percent, then that can increase your fuel costs by 3 percent to 5 percent,” says Cara Junkins with Continental Tire. To keep your tires properly inflated, Junkins suggests checking your tires daily with a calibrated gauge. “But make sure you understand what your typical hauls are, especially if you’re not hauling your usual haul,” Junkins says. “If you’re usually maxed out, and this time you’re running a light load, make sure your tires are inflated appropriately.”

    TMC says spec’ing rib-tread, drive-axle tires instead of lug tread can improve fuel efficiency 2 percent to 4 percent, while using ribs on the drive axles and shallow ribs on the trailer can brighten the picture from 6 percent to 14 percent.

    Junkins recommends paying attention to tread depth and tread compound. “With more rubber comes more heat, but a way to make up for that heat is to pay attention to the tread compound, which can make up for some of that fuel lost,” she says. “There are tires out there that are optimized so that you get the best of both worlds.”

    Don’t neglect wheel alignment, either. Even a slight misalignment, say tire manufacturers, can result in 1 percent to 2 percent fuel economy penalty. Check alignment according to manufacturers’ recommendations or if it feels like the truck pulls to one side.


    Spec’ing your powertrain with a “gear fast, run slow” approach can maximize fuel economy and protect your investment in your equipment. This means the powertrain is spec’ed for a high top speed, but you run at generally slower speeds, thereby keeping the engine in an efficient, low-rpm range.

    Kenworth officials say fuel economy may fall 10 percent to 15 percent if a gear set is selected that has the truck cruise at the wrong rpm. Make sure you choose a transmission and rear axle to match the engine and load requirements for the specific application.

    The new EGR engines make it even more important to spec for fuel efficiency because manufacturers say fuel consumption is 3 percent to 5 percent over comparable late-model engines. With any engine, changing air filters and fuel filters as recommended by the engine manufacturer will help maintain optimal fuel economy.

    Spec’ing for lighter weight, of course, reduces the amount of horsepower required. Several hundred pounds can be saved by spec’ing the lightest weight suspension that will support the application.

    Always use engine, transmission or axle lubes matching the viscosity recommended for specific equipment and operating conditions.

    You can improve your fuel usage without a tremendous amount of work or expense. And even though it might be necessary to cheat on your fuel-efficient diet once in a while, you can still make significant progress if you change certain habits in the long run.
    “You’ve got to pay attention to your mpg nowadays,” says produce hauler Bob Schenkenberg of Bottineau, N.D. “You just can’t afford not to.”