Torque Target

John Baxter | August 07, 2011

He says Eaton has tested drivetrains, even those running direct-drive axles, which can carry much more torque, at cruise rpms as low as 1,100.

Transmission options

Additional steps can be taken to increase fuel savings. One is to spec a 13-speed or 18-speed transmission instead of a 10-speed.

Eaton’s Shane Groner says a 13- or 18-speed can “keep the rpm right where it’s supposed to be.” These transmissions have a 17 percent gear step instead of the 34 percent step of a typical 10 speed. Availability of multi-torque models that give the driver extra zoom only in the top gear and one split down enhance the potential of the devices.

Detroit Diesel’s Chuck Blake notes the challenge is keeping the operator from using the split to downshift early and use more engine rpm than necessary. But when you’re on a steep enough grade to carry you too far below peak torque and you need to shift, “instead of shifting down a whole gear, you can split down and get half a gear,” he says.

Groner says these transmissions have a higher resale value. Eaton has a web-based training video that shows how to effectively use multi-speed gearboxes to enhance rather than diminish fuel economy. (Search “Super 18” on for the links to the main product page and the video). Mack’s Dave McKenna says if your cruise speeds vary widely, a 13- or 18-speed transmission could be ideal. Both Mack’s m-Drive and Volvo’s i-Shift automated transmissions have 12 speeds. This reduces the typical 37 percent drop in rpm after an upshift, which occurs with a 10-speed, to a mere 28 percent, notes Volvo’s Ed Saxman.

“This gives the computer great flexibility in skip shifting, in effect allowing it to bite off a step and a half of what you’d get shifting one gear in a 10-speed,” he says. Such shifting keeps rpm low, yet by reducing the number of times the turbo needs to spin up, speeds acceleration.

McKenna says the 12-speed m-Drive will downshift under cruise conditions at around 1,280 rpm and hold an rpm at around 1,325.

Direct drive transmissions save fuel because the power goes directly through the gearbox without passing through any gear meshes, which reduces friction. The gearing also turns more slowly, which reduces churning of the oil, a source of energy loss. Blake says, “Direct drive gives you a 2 percent to 3 percent increase, or 0.1 mpg, partly because you’re in top gear (direct) over 80 percent of the time.”

The manufacturers’ reps say you need to pick a transmission like an FR series Fuller, rather than the more standard RT, for direct drive, with a more powerful reverse gear, so you can still crawl backwards with the clutch engaged.

Dana’s Steve Slesinski says, “Overdrive transmissions are still the most popular, but some are using direct drive to save fuel.” A typical axle ratio for a direct drive setup would be 2.64:1, he says. Because overdrive ratios substitute driveshaft rpm for torque, you’ll need a higher-torque driveshaft for use with direct drive. “We design the axles to handle the extra torque,” he notes.

Slesinski suggests choosing a 6×2 arrangement where you get the same suspension and tandem axles, but only the forward axle drives. You’ll get an additional 2 percent to 3 percent fuel improvement because of reduced friction, he says.