Downspeeding is one of the primary powertrain-focused strategies for improving fuel economy in heavy-duty trucks necessitated by greenhouse gas emissions regulations. With downspeeding the rear gear ratio is sped up in order to lower the speed of the engine, bringing fuel efficiency improvements. Downspeeding allows the engine to operate at the most efficient rpm while generating the minimal horsepower required to maintain a 65 mph cruise speed.
- 2–3% when implemented alone
- 3–6% when combined with other powertrain technologies
- Due to the lower engine speed
- Downsped axles, being less flexible, are much more sensitive to sudden shocks
- Need to spec an automated transmission with a higher output torque capability, a certain clutch with a higher capacity damper, and higher torque driveshafts and rear axles
1. Allison: The Allison TC-10 10-speed automatic was designed for downspeeding. Its use of a torque converter allows a relatively fast first gear ratio of only 7.4:1. 2. Dana: Dana designed the Spicer® AdvanTEK® 40 Tandem Axle for downspeeding and has rear-axle ratios as fast as 2.26:1. Dana’s SPL-350 main driveshaft and mating SPL-250 inter-axle shaft are specifically designed to supply greater torque capacity, durability, and savings for the demands of certain configurations. 3. Detroit/Daimler: Daimler Trucks provides downsped Detroit powertrains that use the 12-speed DD12 transmission and Detroit proprietary drive axles. Detroit’s own direct-drive downspeeding option is currently limited to a 1200-rpm cruise and 2.85:1 rears. 4. Eaton/Cummins: The SmartAdvantage combines Eaton’s Advantage transmission and Cummins’ ISX engines with various driveshaft and axle choices. Eaton also offers the Fuller Advantage which can be paired with the Cummins MX-13 or ISX engine. 5. Mack: The Super-Econodyne integrates the Mack MP engine, the Mack mDRIVE 12-speed automated manual transmission, and a heavy-duty driveshaft. 6. Meritor: Meritor’s downspeeding strategy includes the MT-14X rear drive axles and the Permalube RPL Series drivelines, designed for the benefits of downspeeding today’s over-the-road trucks. 7. Navistar: International MaxxForce downsped engines have an Eaton clutch, Eaton’s Advantage transmission, and Meritor 14X drive axles. 8. PACCAR: Fuller Advantage is combined with the MX engine in Kenworth and Peterbilt truck brands. 9. Volvo: The Volvo XE combines an I-Shift transmission and a custom-designed 17-in. clutch with an organic lining. Both Dana or Meritor driveshafts and rear axles carry the drive to the rear wheels.
Downspeeding can be adopted in one of two configurations: A direct drive transmission and a fast axle ratio The fast axle ratio direct drive transmission specification is optimal in terms of friction and fuel consumption reduction, but given the potential for driveline failure some manufacturers only approve it for true line haul applications where the highest torque conditions occur infrequently. An overdrive transmission and somewhat slower rear axles. The somewhat slower axle ratio of an overdrive transmission spec subjects the transmission output, driveshafts, and axles to far less torque. This makes it appropriate for both line haul and regional and city delivery applications. An automated transmission should be spec’d on any downsped engine. If a manual transmission is selected, the engine’s torque response during clutch engagement must be specifically tailored to the drivetrain.
“Drivers transitioning directly from a manual transmission to a downspeeding driveline must get used to not shifting and allowing the driveline to do the work. Of course, this probably is easier said than done for a driver who has upshifted, downshifted, and double-clutched for years. They must also adapt to the transmission shifting gears at engine rpms of approximately 1250 rpm instead of manual shifts at around 1800 rpm.” — John Moore, Volvo Trucks “An improvement of 3–6% is what you’ll see; I’m very comfortable predicting an improvement of 3%. When downspeeding is combined with other improvements in fuel economy-related aspects of design, fleets could reasonably be expected to see as much as a half mile per gallon.” — Aaron Peterson, Navistar
“The trend toward engine downspeeding will continue to gain momentum as OEMs look for ways to further improve fuel economy.” — Andy Nieman, Dana Commercial Vehicle Technology
“I hadn’t driven one of these new trucks in a few years and was amazed at how fun they are to drive! And it starts with how quiet they are.” — A major fleet manager “I was blown away by how it drove. In top gear, it would run down to 800–900 rpms, hanging in there very well. One advantage is that it’s very quiet, the engine is not roaring. Drivers like it. They worry about getting up to speed, and it does that okay. We’re not sure whether or not it does have the power to do the mountains, though.” — Phil Braker, Nussbaum Transportation