ASHEVILLE, N.C. — I have developed a new appreciation for drivers who post consistently high fuel economy numbers, especially those who push every day to crack 10 mpg (23.5 L/100km). A lot of discipline and hard work goes into that kind of performance, but the trucks they drive also deserve a good chunk of the credit.
Back in September, Mack Trucks invited a handful of truck writers on a three-day adventure called the 2018 Mack Trucks Performance Tour: Gears, Guitars and Grub. It was intended as a demonstration of the latest Mack engine technology, but it quickly turned into a fuel economy contest of sorts between the CDL-holding editors: Steve Bouchard of Transport Routier, Jason Cannon of Overdrive, James Menzies of Truck News, and yours truly.
As technology demonstrations go, this one was both fun and revealing. Mack assembled a fleet of six trucks for the tour, two each of a particular pedigree. Journalists drove a different truck on each leg of the trip to get a feel for the technology and post their best fuel economy numbers. As I’ll explain, the performance of each of the truck pairs turned out to be similar, regardless of whose foot was on the pedal. That showed me there’s a level of consistency across the technology-matched trucks when operated in cruise control where the truck and the onboard systems are making most of the decisions.
I sort of tipped things on their ear by not using the cruise control and managing the driving process myself. That approach netted me the high mpg for the trip at 9.5 mpg (24.8 L/100 km), and the best out of three drivers on one of the other trucks we drove at 8.9 mpg (26.4 L/100 km). Full disclosure here: the truck I was driving on Day 3, one of the baseline models, broke down so I couldn’t complete the mission. I managed 10.4 mpg (22.6 L/100 km) on one of the most fuel-efficient trucks in the test, Truck 4. My numbers for that trip were looking good before a low-fuel pressure warning shut down the truck.
It was later discovered that a “faulty fitting in the fuel supply system leaked, causing air to be drawn into the system,” Mack told me.
With all the trucks proving themselves capable of at least 8 mpg, I believe the upward variations from 8 mpg (29.4 L/100 km) came from the drivers. This exercise confirms that technology can make an average driver good and a good driver better.
Please don’t misunderstand me here. I’m classifying my CDL-holding colleagues as average drivers only because they do not have the 20 years of experience that I have. And I’m not classifying myself as a good driver just because I got the high average fuel economy numbers for the trip. I would say that I probably have more experience coaxing extra fuel economy from a truck than they do.
In talking with the other drivers, I learned that they used cruise control more than 80% of the time, letting the truck make most of the fueling decisions. My time in cruise was just over 30%. I was actively managing the throttle and the gear selection, and I think that’s how I managed to keep my numbers up.
I made good use of the driver’s display screen that showed percentage of engine load. In days gone by, that would have been analogous to either the pyrometer (exhaust temperature) or an turbo boost gauge (intake manifold pressure). Keeping any of those readings as low as possible means you’re saving fuel. In the case of the engine load display, when the cruise control was engaged, the engine would spool up to 100% when climbing and maintain 100% until the truck got to the top. By modulating the throttle and trying to keep the engine load below 80%, and then backing out of the throttle as the grade leveled out near the top, I’d go over the top at 80 km/h mph rather than 95, for example. Gravity did all the work getting me back up to speed on the way down.
I ran a few uphill segments with the cruise control on, and it appears Mack has set some parameters to keep the engine in low power demand for a longer period of time than traditional cruise control would, but it always hit 100% at some point in the climb.
I’m talking rolling hills here, not climbing mountains. On long grades, 100% power was the order of the day. But when I had a chance to roll back the power a little on a hill, I did it religiously.
And because I would crest the hill at a lower speed, I didn’t hit the engine-brake-on trigger speed quite so soon on the way down, which gave me more opportunity to use gravity to keep fuel consumption down. Mack does have a variable engine-brake-on speed setting that the driver can set to provide more free-rolling time.
About the only time I engaged the cruise control was to coax the transmission to drop out of gear for a little free-rolling time.
The “problem” with cruise control is that it wants to maintain a set road speed, so it fuels the engine and shifts the transmission accordingly. Mack’s GPS-based predictive cruise control system does a pretty effective job of managing road speed based on terrain, but while controlled the fueling and the shifting, I could get the engine revs to drop lower than the cruise control wanted to go.
For example, on the two HE-equipped trucks I drove, and particularly the 6×2 with the 415/1,760 MP8 engine, I had the revs as low as 875 a few times climbing a hill, just by feathering the pedal and trying to prevent a downshift. I’d be the first to admit a truck that was programmed to drive like that would probably annoy most drivers, but driving for maximum fuel economy was the exercise here. Frankly, I can’t understand why any driver who buys their own fuel wouldn’t drive like that.
The clincher was on that 300-km leg, I arrived at the destination just 10 minutes behind everyone else, but at 10.4 mpg I beat most of the other trucks by a full 2 mpg, and the second-best truck by 1.5 mpg. I have always said the winner of the game is not the driver who gets to the bank first on payday, but who gets there with the biggest deposit.
Each pair of trucks was identical in most of the componentry, including the tires, brakes, fuel tanks, and interior packages. Each featured an MP8 engine and an mDrive 12-speed overdrive (0.78:1) automated manual transmission, but the engines spec’ varied.
Trucks 1 and 2 (the baseline models) had the full Anthem aero package, while Trucks 3 and 4 and Trucks 5 and 6 had the HE+ (high-efficiency) aero package, which included a roof fairing with trim tab, extended side fairings, chassis fairings with ground effects, and an aero bumper with a spoiler.
Tractors 1 and 2 (red): Mack Anthem 70-inch standup sleeper 6×4
Engine: Mack MP8-445C, 445/1,860
Rear axle ratio: 2.47:1
Tractors 3 and 4 (white): Mack Anthem 70-inch standup sleeper 6×2 liftable pusher axle
Engine Mack MP8HE-415SE, 415/1,760 with ERT
Rear Axle Ratio: 2.50:1
Tractors 5 and 6 (blue): Mack Anthem 70-inch standup sleeper 6×4
Engine: Mack MP8HE-445SE, 445/1,860 with ERT
Rear Axle Ratio: 2.47:1
The Mack MP8HE engine employs Mack’s Energy Recovery Technology, which is essentially a turbo compound system that captures energy from the engine’s exhaust and delivers it back to the engine crankshaft through a gear train as additional torque, Mack says.
I believe the HE feature was instrumental in bumping up the fuel economy number on the trucks equipped with it as it allowed lower engine speeds than the non-HE MP8.
10.4 mpg explained
The trip logs from the second leg of our trip, from Nashville to Memphis, Tenn., showed I hit 10.4 mpg (22.6 L/100 km) for the day. A day earlier, my colleague Steve Bouchard of Transport Routier managed 9.8 (24) with the same truck, while Truck News’ James Menzies posted 9.8 (24) on Day 3. Those numbers are remarkable, but I think they need a little perspective.
We spent most of the run in the top gears where the engine is most efficient, and only a few stretches wheeling through town and in parking lots in the lower gears. I saw numbers like 2.5, 4.0 and maybe 5.0 mpg on the screen. That’s a respective 94, 58.8 and 47 L/100 km if you prefer. To illustrate the point, as I exited the highway at Memphis and headed for the dealership there, the screen showed 10.6 mpg (22.2 L/100 km). Upon shutting the truck off at the dealer two miles later it was down to 10.4 (22.6 L/100 km).
The numbers revealed here are really just for a single leg of a trip with very few off-highway miles, idling at traffic lights, and looking for parking. Drivers who can post fuel economy numbers in the high eights, nines or even 10s over a full month are truly heroic and extremely disciplined operators.
That said, any truck that will cruise at 8.5. 9.0 or even 10.4 mpg – 27.7, 26 or 22.6 L/100 km — is doing something right. The baseline trucks in our test averaged 8.0 mpg (29.4 L/100 km) over the three-day 1,480-km trip. The blue trucks averaged 8.4 mpg or 28 L/100 km (4% better than the baseline), and the white trucks managed 9.2 mpg or 25.6 L/100 km. The white trucks were spec’d to be the most fuel efficient with a combination of a 6×2 powertrain, a lighter engine spec’, Mack’s Energy Recovery Technology (turbo compounding), and the advanced aero package. They clocked in 14% more efficient than the baseline trucks.
Curiously, Truck 4 was consistently 1.4 mpg (168 L/100 km) better than Truck 3, yet the two were identically spec’d and the fuel-burned and distance-run numbers all work. I was lucky and recorded my 10.4 mpg (22.6 L/100 km) leg on Truck 4. The other drivers also recorded their highest scores on Truck 4. That’s a huge gap between two identical trucks. It could be explained by the fact that the trailer tails did not automatically deploy on some trucks on some legs, but I’m not aware which trucks were affected. There may also have been differences in the tractor/trailer gap or the tire inflation pressures. The average speeds for the two trucks over the trip were similar, 83 and 84.5 km/h, with Truck 4 posting the lower average speed. Could that account for a 1.4 mpg (168 L/100 km) difference?
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