Cruising down the Interstate, you glance at the tachometer and gasp as you see the needle hovering at under 1,200 rpm. Yet the engine isn't vibrating or straining, and the rig continues to move steadily forward.

The mDrive automated transmission upshifts early - at 1,400 or so with a light foot on the accelerator. You'd never ask an engine to do this - but Mack has with its new Super Econodyne option.

Super Econodyne knocks about 200 rpm off normal cruising speed of the 13-liter MP8 diesel, which theoretically improves fuel economy by about 2% compared to standard, faster revs, and fleet tests show higher gains. "Short shifting" at low rpms contributes to the savings. The idea is to let the engine lug and use its torque, rather than horsepower produced at higher revs, to propel the truck. This saves fuel by reducing friction from internal moving parts.

Last year Volvo, Mack's sister company, introduced its XE (for exceptional efficiency) feature, also using a 13-liter diesel and its I-Shift automated transmission, which operate similarly. Super Econodyne, or SE, is Mack's version. While the mechanical components of the engine and transmission are the same as Volvo's, Mack's software is altered to achieve operating characteristics wanted by Mack customers, says Dave McKenna, director of powertrain sales and marketing.

With a heavy foot, the rpms go higher during acceleration, to 1,600 or so, and you can force even faster revving by playing with the buttons on the selector. But if you lighten up and leave things alone, the rig accelerates and cruises well, and you soon learn to concentrate on steering and otherwise managing the rig in traffic.

Addressing skeptics

Drivers who do things the wrong way can kill fuel efficiency, but SE takes away those bad decisions, McKenna says. In his 39 years with the company, he has seen the old high-torque-rise Maxidyne and many other iterations on the quest for top fuel economy. And he knows that properly engineered, a diesel can lug down low without its valves burning or pistons cracking or any other damage.

"Nobody wants to run at 1,160 rpm," he says of disbelieving drivers and owners. "They'll say, 'I can't do that. I can't do that.' I'll tell you what: It's my engine. If it blows up, I'll rebuild it." The warranty on the engine, by the way, is two years or 250,000 miles.

To encourage skeptics to try the concept, Mack has made Super Econodyne a no-cost option for anyone buying its all-Mack components: a 13-liter MP8SE engine set for 455 horsepower and 1,750 pounds-feet; a 12-speed mDrive automated mechanical transmission with a 0.78 overdrive top gear; C125-series double-reduction tandem rear axles with a 2.66 ratio; and 11R22.5 tires (though demo tractors had wide-base single rear tires and wheels). This is a gear-fast, run-slow specification; while the gearing allows a top speed of 95 mph, the SE's electronic controls were programmed to enforce the "slow" - 70 mph while in cruise control and 72 mph tops.

McKenna explained the SE option during a mid-June press event at Mack's old technical center, now its Customer Service Center, in Allentown, Pa. Super Econodyne is a highway tool and is not meant for vocational trucks, though haulers of bulk cement, rock and building supplies might well use it. Later, there will be an 11-liter SE weighing about 300 pounds less than this 13-liter model, McKenna said.

On the road

After a briefing, writers who had never driven a tractor-trailer could safely take the two demonstration rigs onto the adjacent 3/4 mile test oval because the mDrive self-shifting trannies made driving easy. A few of us have commercial licenses and experience with tractor-trailers, so were qualified to go out onto nearby Interstate 78. After about 50 miles on the road, I came away impressed with the Super Econodyne's operation and the quietness and comfort of the Pinnacle tractor.

Actually, I drove both demo tractors, one hitched to a 48-foot flatbed laden with heavy concrete blocks and the other pulling a 53-foot van with a similar load for ballast. Each rig weighed close to the 80,000 pounds for which the SE system is rated, so the MP8 engines had to work to keep us moving. I took both around the test track, watching the SE short-shifting the tranny. I learned that I could punch the PERF (for performance) button on the "premium" keypad and let revs build, though this was needless on level pavement.

A "fleet" keypad option omits PERF and up-and down-arrow buttons, taking away such choices and forcing a driver to let the SE work on its own. This is fine under many circumstances, but safety-conscious drivers want to downshift on downgrades to get more retarding power from the engine brake, and they won't be able to with the standard selector.

When I was finished on the track, my guide, Powertrain Sales Manager Joe Scarnec-chia, directed me out onto city streets toward 1-78. "You turn right just up here," he said, and I did - one corner too soon, which caused me to blunder into a Home Depot complex. I headed toward the rear of the building intending to go around the back, only to find a rig blocking the entire alley. So I backed up a short block and maneuvered through the parking lot to get us to the on ramp. The mDrive's automated clutch with its smooth-engaging organic facing and the setback-axle Pinnacle's tight turning ability made it all rather easy.

Finally on westbound 1-78, I watched the tach as the short-shifting continued, even in high-range gears as we got up to cruising speed. Just as McKenna had said, the revs were low, about 1,100 at 60 mph and 1,200 at 66. Strong torque kept us well propelled on the level stretches, but on moderate upgrades road speed usually fell off by 5 mph, just as it would when you keep a manual tranny in high gear while climbing hills. To maintain speed on hills you need horsepower that only comes from higher revs. I got this by either stomping on the accelerator or hitting PERF and hoping the mDrive would react with a downshift, which it usually did.

In the olden days, low-rev running was sometimes accompanied by vibrations and damage to driveline components, but engineers have long learned to elimi nate those. So the Super Econodyne powertrain can be expected to deliver smooth and quiet performance along with better fuel economy - 5% to 10% better in customer tests, McKenna said. It'll take operators some getting used to, but they'll end up with a relaxed driving experience they might grow to like.

Truck: 2013 Mack CXU613 Pinnacle road tractor w/ setback steer axle, Rawhide trim package, 70-inch Hi-Rise sleeper box, BBC 173 inches, GVW 49,200 lbs., GCW 80,000 lbs.
Engine: 13-liter Mack MP8-445SE, 445 hp @ 1,500-1,800 rpm, 1,750 Ib-ft. @ 1,080-1,380 rpm, w/ PowerLeash engine brake, Bendix Wingman adaptive cruise control and Borg-Warner multi-speed viscous fan drive
Transmission: Mack mDrive 12-speed automated Mechanical w/ 17-inch organic-facing automated clutch, 0.78 overdrive and Grade Gripper hill holder
Steer axle: 12,000-lb. Mack FXL12 on taperleafs
Tandem: 40,000-lb. Mack S125/126 double-reduction w/ 2.66 ratio, on Mack AL401 air-ride
Wheelbase: 215 inches
Fifth wheel: Jost JSK37UWL w/ 24-in. slider
Tires & wheels: 275/80R22.5 Michelin XZAG+ front, 445/50R22.5 Michelin X-OneXDA rear, on Alcoa polished aluminum discs
Brakes: Bendix S-cam drum w/ Bendix ABS
Fuel tanks: Twin 117-gal. aluminum
Demo trailers: 48-foot flatbed and 53-foot van

From the October 2012 issue of HDT.