Executive Summary

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.

Downspeeding
Available Systems

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.

Manufacturers
Common Fleet Strategies

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.

What Others Are Saying
Decision Making Tools

Decision-Making Tools

Conclusions
  • Downspeeding is an idea that has been around a long time, but recent complementary technologies have made it more attractive and practical.
  • Fleets in long haul operation should consider downspeeding in their powertrain for improved fuel efficiency and driver satisfaction.
  • Some manufacturers are already at work on aggressive downspeeding with rear axle ratios at 2.28:1 or lower.