Swihart continues, “Downspeeding is one of the options and will take into account the engine requirements based on what is selected.”
As for concerns regarding long-term torque stress on drivetrains, he says that Kenworth has conducted extensive testing in this area and has taken the necessary steps to alleviate any concerns. Downspeeding does introduce more torque into the drivetrain. As a result, spec’ing a truck with a downsped optimization goal will force higher strength drivetrain components onto the sales order.
Owners’ responsibility
“It’s important to remember that every owner needs to look at and discuss with their dealer what is best for their application, according to the payloads they carry and the routes they run,” says Stu Russoli, Mack’s highway and powertrain products marketing manager. “With that said, optimized speeds are anywhere from 60 to 68 MPH, depending on rear axle ratio and tire size. We are currently using ratios from the 2.51:1 range up to 2.83:1, depending on whether Mack axles or supplier axles are spec’d, and on tire size. The target RPM for the best blend of fuel efficiency and performance is around 1,150 RPM, plus or minus 50 RPM. The optimum speed for any one set-up then falls within a 4 MPH range.”
Russoli goes on to say that working with fleets to achieve the best results requires asking customers questions to better understand their application, including:
• What is payload are they hauling?
• What is a typical trip distance and route topography?
• Do loads reach maximum gross weight allowed (grossing out)?
• Do they haul below the maximum weight, but fill the cubic footage of the trailer?
• Is the truck loaded for both directions or only one direction, or is it a diminishing load?
“Each customer has different needs and may use trucks in a different manner so it’s important to determine this information up front,” Russoli adds. “We feel our integrated powertrain—Mack MP engines, mDRIVE transmission and Mack rear axles—is the most efficient. The components are designed to work together, which means our electronic control units [ECUs] share 100% of the data with each other 100% of the time. Our MP8-445SE Super Econodyne package, which includes the mDRIVE transmission and Mack 125/126 carriers, provides 445 HP and 1,760 lb.-ft. of torque, along with a downspeeding strategy for superior fuel economy. For applications requiring a lighter weight, the MP7-405SE package with our 11-liter, 405 HP engine, is the preferred choice.”
Mack continues to develop strategies for increasing fuel economy by downspeeding engines and tweaking axle rations. “Always check with your dealership to determine the latest recommended combinations,” Russoli says. “Also, downspeeding is not for everyone or every application. It is important to talk with a knowledgeable salesperson and ask questions, make sure you have that 4 MPH speed range set to the speeds the trucks will be traveling, and check your startability and gradeability.”
