The Low Down On Downspeeding

Nov. 23, 2015
One of the primary powertrain focused strategies for improving fuel economy is downspeeding.

One of the primary powertrain focused strategies for improving fuel economy is downspeeding. There have been lots of new developments in this area as truck makers and component suppliers are looking for ways to lower engine speed to improve fuel economy.

We recently released a Confidence Report on downspeeding to present a full view of the technology, see what products were on the market, and confirm benefits and challenges.

Our research found that downspeeding could result in a 2% to 3% fuel savings. We also found that downspeeding is at a tipping point with rear axle ratios of 2.47:1 and engine rpms of 1100 to 1300. We discovered that aggressive downspeeding with rear axle ratios of around 2.08:1 and engine cruise speeds of 900 to 1000 are just around the corner.

Downspeeding is most suitable for long haul tractor-trailers and regional haul day cabs. Basically it works best for trucks with high mileage with most miles driven on highways and very infrequent pickups and deliveries.

If you’ve heard about downspeeding but aren’t sure if it’s right for you here’s something to remember. You need to be clear about the use of the vehicle when spec’ing with a downsped drivetrain. It is a risk to spec a super fast axle when the truck sees frequent starts from 0 mph even for a small percentage of its running time.

The other key point is that you have to make sure to look at the powertrain as a whole and pay attention to things like engine parameters, whether the transmission is electronically controlled, the driveline, the rear axle, the engine and even the tires. It is extremely important to work with your OEM to specify the necessary components to support a downsped engine.

The upcoming Phase 2 of GHG emissions levels is going to make downspeeding even more attractive as manufacturers look for ways to meet the improved fuel economy standards.

The bottom line is if your fleet has long duty cycles, you should consider downspeeding your powertrain.  But this is one of those times when you must work closely with your OEM and powertrain component suppliers to make sure the truck is set up properly or you risk driveline failure. So be prepared to pay a little more for more robust components. But remember the 2% to 3% fuel savings you’ll reap from downspeeding means you’ll recoup the extra expense in less than a year.

To learn more about downspeeding and some of the other technologies that we have studied like idle reduction, electronic engine parameters, automated transmissions, low rolling resistance tires, lightweighting, downspeeding and 6x2 axles join us on December 9 at the Automotive Research Center of Indianapolis for a trucking efficiency workshop. See you there.

About the Author

Michael Roeth | Executive Director

Michael Roeth has worked in the commercial vehicle industry for nearly 30 years, most recently as executive director of the North American Council for Freight Efficiency (NACFE). He serves on the second National Academy of Sciences Committee on Technologies and Approaches for Reducing the Fuel Consumption of Medium and Heavy-Duty Vehicles and has held various positions in engineering, quality, sales, and plant management with Navistar and Behr/Cummins.

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