Direct drive transmissions are well suited to relatively flat on-highway applications .  Photo: Eaton

Direct drive transmissions are well suited to relatively flat on-highway applications. Photo: Eaton

The long-running debate over which type of transmission — direct drive or overdrive — provides the greatest efficiency is one that’s not easily settled and not likely to go away.

Conventional wisdom suggests direct drive transmissions, with their ability to transmit power directly through the main shaft with minimal parasitic losses, should be more efficient – and sometimes that’s true. If only it were so simple.

In reality, there are many variables to consider when deciding whether your fleet will be best served by a direct drive or overdrive transmission.

“Direct drive and overdrive transmissions have existed side by side for some time, but there’s certainly a lot of confusion as to which gives you the greatest efficiency,” says Evan Vijithakumara, Eaton’s global product strategy manager for heavy-duty transmissions.

Application, application, application

It all depends on the application. Generally speaking, in ideal conditions running on-highway over relatively flat ground, direct drive is the more efficient option. But throw in some hills, higher average road speeds and some off-road requirements, and overdrive becomes a better fit.

Vijithakumara warns that while it’s true that a direct drive transmission itself will operate more efficiently in top gear, that doesn’t always translate into greater fuel economy.

“While direct drive can be more efficient as a transmission, that doesn’t guarantee it will be more efficient for the system – the system being the engine, transmission, axle and tire combination,” Vijithakumara explains.

Routes that include harsh environments and hilly terrain are best suited to overdrive transmissions. Photo: Eaton

Routes that include harsh environments and hilly terrain are best suited to overdrive transmissions. Photo: Eaton

When in top gear, direct drive transmissions do not transmit torque down through the countershaft, so all the power generated by the engine is passed directly through the input shaft into the output shaft.

“We consider that a ‘no-mesh’ condition and when you have no loaded gear meshes, the transmission has reduced parasitic loss, resulting in more efficient operation,” Vijithakumara says.

On the other hand, the higher front box ratios inherent in direct drive transmissions have implications on downstream components that must be considered during the spec’ing process.

“With a direct drive transmission, the torque is passed straight through to the driveline, so an adverse event such as moving from an icy road to dry pavement can transmit a lot of force further downstream through the driveshaft and U-joints, which could be potentially damaging," Vijithakumara says. "On an overdrive transmission, you have torque being transmitted through the countershaft gears, so you’re not effectively bypassing the transmission. When a harsh event occurs, the additional backlash and elasticity within the transmission can prevent damage downstream.”

The complete system

Interest in direct drive transmissions has increased in recent years, as fleets explore all available options to increase their efficiency. But the reality is that in North America, high average road speeds and hilly, even mountainous, terrain often make overdrive transmissions a more appropriate solution. It all depends on application and duty cycle.

“I think there is a misperception that a single product can fit all applications,” he adds. “It really depends on what you’re trying to do.”

Overdrive transmissions, with their higher overall ratios, provide greater startability and gradeability, as well as lower transmission sump temperatures in hilly terrain. The contribution this makes towards total vehicle efficiency should not be overlooked. In addition, trucks equipped with overdrive transmissions have more application flexibility which can help with vehicle resale compared to a direct drive configuration tailored for a specific route.

Overdrive transmissions also allow for the greater use of downspeeding (cruising at lower engine rpms), which can drive fuel savings.

Eaton says its new Fuller Advantage Series automated transmission, available as part of the SmartAdvantage Powertrain with the Cummins ISX15, marries the benefits of direct drive transmissions with those offered in an overdrive configuration. (Available with the Paccar MX-13 engine in the Apex package at Peterbilt, the Kenworth T680 Advantage package, and at International paired to the N13 engine.)

In recent years, the interplay between the transmission, engine and axles has increased, meaning it’s more important than ever for fleets to take a holistic approach to spec’ing the powertrain as a complete system. When evaluating transmissions for fit, all parts of the system do factor in.

Direct drive and overdrive transmissions can be equally robust and reliable, but to ensure the best performance possible it’s important to consider every aspect of your application. Consult with the experts and then specify accordingly.

Adapted from a "Truck Experts Article" on the Eaton website. Used with permission. This article was selected and edited by HDT's editors to provide useful information to our readers.