Downspeeding: What You Need to Know

Downspeeding is no doubt here to stay as a strategy to increase fuel efficiency and cut back on negative environmental impacts from burning that fuel. Downspeeding accomplishes this by running engines slower at cruising speeds. But this significantly affects how Class 8 trucks function. If you don’t understand what’s different – and how to proactively address those issues – you could easily wind up with fractured components. No savings there.

Downspeeding really can lower your fuel consumption, by as much as 1-2%. That can add up nicely over time. However, the mechanics of downspeeding can put your drivetrain under far greater stress from the increased torque. Once you understand how the process works, you’ll be well-informed to choose appropriate downspeeding applications and the right components to protect your drivetrain from torque overstress. That’s what causes fracturing.

You can downspeed your powertrain in two ways. Combining a fast axle ratio with direct drive transmission enables you to generate higher engine torque. That requires drivetrain components capable of withstanding that high stress. If you’re using a slower axle ratio and an overdrive transmission, you can choose different components.


EPA 2010 engines have substantially higher peak cylinder pressure and power density than older designs. When the clutch is engaged, the engine reaches full-rated engine torque much faster. Torque can rise from 700 RPM to 900 RPM is less than a half-second. This feels very different to drivers used to older engines — manual transmissions are harder to handle, and automated manual transmissions are more limited. You need a dual clutch. Stress increases on all components of the rear drive axle and drive shafts.

But there are distinct benefits. Using a faster numeric rear drive axle ratio (under 2.64) with a direct drive transmission provides both excellent power for start-up, trailer tug and trailer positioning and reduced RPM at cruising speed for greater fuel-efficiency.

Testing reveals optimum solutions

In order to prevent torque overstress in downspeeding applications, Meritor conducted an extensive study of the effects of engine downspeeding on drive train components. The study included both dynamic simulation testing and live vehicle testing. Researchers could calculate theoretical maximum drive train torque values, but how would they compare to measured test values?

Under dynamic simulation, tests revealed that the faster the rear axle ratio, the faster peak transient torque increases in response to decreasing engine speed response time. For example:

  • When decreasing rear axle ratios from 3.55 to 2.85, engine ramp-up time dropped from 2 seconds to 0.5 seconds. At the same time, the percentage of peak transient drivetrain torque increased pretty evenly.
  • When decreasing rear axle ratios from 2.79 to 2.19, engine ramp-up time followed the same pattern – dropping from 2 seconds to a half-second. However, peak transient drivetrain torque increased at a much faster rate.

It’s this percent increase of peak transient drivetrain torque that can damage transmissions, drive shafts and rear axle components.

Researchers used several combinations of truck chassis and engines to conduct their live tests. Each had a manual transmission and a rear axle ratio under 3.0. They tested trucks under the three working conditions where drivers most often reported drive shaft universal joint fractures:

  • A torque build-up event, in which the driver repeatedly accelerated and braked while rolling slowly.
  • Aggressive clutch release, in 1st, 2nd and reverse.
  • Trailer slide, accelerating then abruptly releasing the clutch while trailer brakes are on.

They found that calculated steady-state torque values were not necessarily sufficient predictors of “safe” values. They noted that “by limiting parameters like low speed torque and engine speed, the overstress drivetrain conditions can be reduced and eliminated.”

Moving forward

Research engineers say the move toward downspeeding means more drive train components will be subjected to potential overstress conditions, increasing risk of fractures. However, manufacturers of engines, trucks and drivetrain components are working together to develop reliable control strategies to prevent problems. They agree that this solution has more value than simply using larger drivetrain components, because that would merely shift the risk of damage onto other components in the system.