When I saw the numbers creeping up into the 10s on a recent test drive in a Peterbilt 579 EPIQ,...

When I saw the numbers creeping up into the 10s on a recent test drive in a Peterbilt 579 EPIQ, I started pushing for 11. And it worked. 

Photos: Jim Park

It's easy to get excited and stay excited about fuel economy when you can see positive change coming from your efforts. When I saw the numbers creeping up into the 10s on a recent test drive in a Peterbilt 579 EPIQ, I started pushing for 11. And it worked.

I never thought I'd be unhappy with 10.2 mpg, but once I had tasted 11 mpg, anything less was disappointing. This test drive of a Peterbilt 579 EPIQ with an UltraLoft sleeper wasn't meant to be a fuel-economy test, but that big average trip fuel economy number on the driver information display quickly became an obsession. I watched it climb through the 7 and 8 mpg and up into the 9s, and once it hit 10 the game was on. I had logged 10.2 miles per gallon on a trip last fall and was determined to beat that.

I had travelled several hundred miles earlier in the day bouncing up and down between 11.0 and 11.1 mpg, so I was pretty disappointed see the final trip average figure had dipped back to only 10.2. Imagine that, only 10.2.

The trip began in Scottsdale, Arizona and ended 1,028 miles later at the Peterbilt factory in Denton, Texas. The truck had been driven to Scottsdale for a sales meeting, I drove it back to Denton in the company of Nick Smith, Peterbilt's public relations manager.

Our route took us on AZ 87 from Scottsdale to Payson, then on AZ 260 and AZ 377 up to Holbrook where we picked up I-40 eastbound. We ran I-40 east to Amarillo, Texas where we swung southeast on US 287 and followed that practically right to the factory.

When we picked up the truck, the trip average fuel economy showed 7.5. Since the first leg of our trip would take us through some mildly mountainous terrain and stop-and-go traffic in Arizona, I wasn't expecting miracles for fuel economy, but I reset the trip fuel economy anyway and hoped for the best. As it turned out, the average on the first leg -- Scottsdale to Santa Rosa, New Mexico -- was 7.8. Better than I had expected. 

I reset again before we left the hotel in Santa Rosa and watched the numbers climb steadily from the low 7s up into the low 10s as we neared Amarillo. Things really got interesting between Amarillo and Memphis, where I started seeing high 10s and up to 11.1 mpg. The display still showed 11 mpg when we stopped for dinner at Vernon. The numbers began to drop after we left Vernon, dipping to 10.4 when I got off US 287 at Decatur. Running the back roads from Decatur into the plant in Denton took my numbers down even further until the trip average read only 10.2 when I turned off the key. I was pretty bummed out about the drop and I have a theory on why it happened, which I'll get to shortly. 

This Peterbilt 579 EPIQ model and the trailer were equipped with all the aero devices...

This Peterbilt 579 EPIQ model and the trailer were equipped with all the aero devices commercially available. 

What Drives 11 mpg?

This truck had much going for it in terms of being about as aerodynamic as commercially possible and it had a modest engine spec (see specs below), meaning digging deep into the torque and horsepower wells wasn't consuming as much fuel as, say, a 500/1850 engine would. We also had a pretty good tailwind that day, probably averaging 20-25 mph from the rear. The terrain was conducive to free-rolling and coasting too. The big benefit of those rolling hills comes from easing up on the throttle during the climb and letting the truck roll out on the down side. It's easy to give up a few mph going uphill because you'll make it back coming down.

My strategy was to use cruise control on flat ground where there fewer opportunities to out-drive cruise, but to turn it off and manage the fueling manually where rolling hills present an opportunity for slower climbs followed by a roll-out. There were lots of those rolling hills between Santa Rosa, NM and Vernon, TX and I tried to take advantage of every one of them.

The Peterbilt dash has a fuel economy gauge that shows when the driver is operating above or below average fuel economy. It displays one or two green bars either side of average. While going up a short hill, I feathered the throttle trying to keep only one bar below average lit, but when using cruise control on a similar hill, two bars were always lit. On top of that, the electronic turbo-boost gauge and the sound of the engine are good indicators of how much fuel is being burned to maintain speed.

Achingly close to 11, and the driving force behind driving as frugally as possible to tip the...

Achingly close to 11, and the driving force behind driving as frugally as possible to tip the number up.

What the Fuel Economy Gauge Tells You

The figure shown on the dash represents a mathematical calculation of fuel economy, not actual gallons flowing through the fuel pump. It's not going to be 100% accurate, but it's generally not far off — maybe a couple of percentage points either way. And it represents, in this case, average rather than instantaneous fuel economy.

If you recall from high school math class, to get an average you add any number of elements, 5 for example, and then divide the sum by the number of elements. For example, 2+3+3+4+5 = 17. Divide 17 by 5 and 3.4 is the average. If you can imagine the truck's ECM taking fuel consumed calculations at one-second intervals, over a minute you'd have 60 values. Your average would be the sum of those numbers divided by 60.

All that to say, if you manage to maintain a high average fuel economy for a long period of time, then the periods where the fuel economy was lower won't have as much of an impact of the final average. It all begins to level out. The downside to that is leaving that average to build over long periods of time -- as in the lifetime average for the truck. The numbers never change. 

As a way of staying engaged and seeing the improvements, it helps to reset the trip fuel economy every day. Doing that keeps the string of data points stored in the memory much shorter, and that allows change to show up on the dash display. When drivers see the numbers change, they realize they can influence the numbers by driving more efficiently. Once drivers can see the difference their driving habits make, the more likely they will be to try to keep those numbers climbing. 

Most trucks have several trip logs, so drivers can set one by the day, one by the week, another for a month, or per trip, etc. 

When we got into the truck in Scottsdale, I wasn't really thinking about fuel economy, but I got...

When we got into the truck in Scottsdale, I wasn't really thinking about fuel economy, but I got the spirit when I started seeing the numbers creeping upward.

Spec'ing for the Application

I don't know what the engineers had in mind when they spec'd this truck, but it was probably a standard spec for a truckload application where payloads are fairly light -- 50,000-70,000 lbs. GCW -- operating on flat to lightly rolling terrain. It was probably spec'd to run 70 or 75 mph because at 65 mph in 12th gear, the engine turned 1,100 rpm, just 100 rpm above the seemingly hardwired downshift point. It turned 1,200 rpm at 70 mph and about 1,275 at 75 mph. That left a little more engine speed latitude before dropping out of peak torque at 950 rpm.

Because I was running below the intended road speed, the transmission tended to shift between 11th and 12th gears at the slightest drop in speed while cruise control was on. When not in cruise the engine speed would dip a little further. It would not go below 950 without a downshift. 

I don't think I would have seen 11.1 mpg if I had set the cruise speed at 75. Still, if the driver was willing to engage and drive the truck for best possible fuel economy, the spec was fine running at 65. It did okay in the mountains too. I was pulling the 7% grades on AZ 87 in 8th or 9th gear at 30-40 mph. With a 500/1,850 engine, I might have gained a gear and a few miles per hour, but it would not have had much of an impact on my trip time. If I ran in the mountains all day long, higher torque and horsepower would be the way to go, along with a 2.90 or maybe a 3.11 rear-end.

This truck was equipped with "Neutral-Coast," where the transmission drops into neutral to virtually eliminate the parasitic drag from the engine while rolling down a slight grade. The cruise had a 65-mph limit, so I couldn't set it higher, as a result the truck had just a 5-mph  neutral-coast window between 65 and 70 mph, after which the transmission would slip back into gear and sometimes downshift to 11th to slow the truck -- it even engaged the engine brake on a few occasions. I don't know if that's an adjustable parameter, but if it is, I'd set a 10-mph coasting window to take fuller advantage of those opportunities.

Keeping the tractor-trailer gap as tight as possible helps, as does the trailer-mounted gap...

Keeping the tractor-trailer gap as tight as possible helps, as does the trailer-mounted gap reducer. 

Driving for Fuel Economy

Driving for maximum fuel efficiency takes some discipline and determination, and a little knowledge about how to coax the best performance from the truck. I find it exciting to get those number going in the right direction, and once you start to see results, it's easier to stay excited.

And I should stress, this was just one leg of a trip, not a monthly average. A typical compliment of local stop and go driving, reversing, idling, etc., would pull that average down pretty quickly over 30 days. But I'm happy to say I broke 11 mpg for a couple of hundred miles that day, and I believe if I was headed out in that truck again tomorrow, I'd be trying just as hard to get back up there again.

As I noted earlier, resetting the trip fuel economy daily makes those changes more obvious, and with a truck like this, it's easy to move the needle up through the 7s and 8s. It gets progressively harder though as you get up into the 10s and maybe higher. I couldn't get higher than 11.1, and I found it took only a bit of bad driving to knock the number down into the 10s (that where the discipline comes in).

As I noted earlier, my fuel economy began to decline after our dinner stop at Vernon. It was dark by then, and couple of things had changed. First, the tailwind had diminished, as it often does at night, so I lost that advantage. The other change was that I could not see as far down to the road in the dark to anticipate the hills. I was still driving manually and trying to coast, back off on the uphill, etc. but it just wasn't working. As the number fell from 11.0 through 10.9 and down to 10.6, I gave up and put the cruise control on and let it manage the fueling. The average had dropped a further two-tenths to 10.4 by the time I got off US 287 at Decatur. The local roads and traffic lights took me down to 10.2 by the time I pulled into the plant at Denton.

I was kind of bummed out about that, but I knew the local driving would have a negative impact on the average. I was trying to keep the average as high as possible while out on the highway, hoping the average would drop no further then maybe 10.5 on the local roads. 

The previous night, by the time we had passed through Albuquerque, it was getting late, I was tired and I was keeping a weather eye on my log. I decided to paste the throttle pedal to the floor and held it at 75 mph all the way over to Santa Rosa. Fuel economy is important, but so was getting to my destination before I turned into a pumpkin.

Unfortunately for a lot of drivers, tight scheduling often gets in the way of better fuel economy. I milked it on the leg between Santa Rosa and Denton because I could. I guess you have to make hay while the sun shines, so take advantage of any opportunity that presents itself, and try running with the cruise control off on the kinds of roads where a bit of fiddling with the throttle pedal can pay off.   


Engine: Paccar MX-13 455 hp / 1,650 lb-ft @ 900 rpm

Transmission: Paccar automated 12-speed with 0.77 OD (11th gear is direct)

Drive axles: Paccar 40,000-lb with a 2.64 ratio.

Steer tires: Bridgestone R 283A 16-ply 295/75R2.5

Drive tires: Bridgestone R 283A 14-ply 295/75R22.5

Aero: EPIQ aero package, including:

  • Full side fairings with rubber extender skirts to the back of the sleeper
  • Closeouts around the front wheel wells
  • Aero-optimized sunvisor
  • Roof fairing close-out and trailer bridge
  • Cab fairings with rubber extenders
  • FlowBelow drive-wheel fairings & aero wheel covers
  • Optimized hood, windshield and cab-roof profile


Side skirts

Boat tail

Aero wheel covers

Mustache-type gap reducer

Rib tires

Gross combination weight: 77,380 pounds

About the author
Jim Park

Jim Park

Equipment Editor

A truck driver and owner-operator for 20 years before becoming a trucking journalist, Jim Park maintains his commercial driver’s license and brings a real-world perspective to Test Drives, as well as to features about equipment spec’ing and trends, maintenance and drivers. His On the Spot videos bring a new dimension to his trucking reporting. And he's the primary host of the HDT Talks Trucking videocast/podcast.

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