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Spec By Numbers

Posted: August 1, 2014 by Jim Park

How well you spec the truck in the first place, and how closely you operate the truck to the spec you laid out can make a real difference. Derek Varley says overbuying means paying twice for the mistake: once on the bill of sale, and again at the pumps. “Spec’ing big power for a tandem application is like buying a $400,000 house and leaving all the windows and doors open all summer with the A/C running full out,” he says.

Varley is the fleet manager at Mackie Moving Systems in Oshawa, Ont., and he’s a student of the art of truck spec’ing — actually, he could be the professor.

He recently spec’d a truck for a tandem run from Oshawa to Pittsburgh, Pa., averaging 35,000 to 40,000 lb payload down bound and 15,000 lb back. The truck has been on the run for a year, and Varley reports it’s doing an impressive 8.5 mpg US. The spec is a Cummins ISX 450, a ZF Meritor FreedomLine 12-speed automated tranny with 0.83 overdrive gearing, and a 3.55:1 rear axle ratio.

Were you to go through your load manifests, you might find that a majority of your loads are similar in weight. It’s not that common to run fully grossed all the time. The route Varley’s truck runs is quite hilly, so the truck is working. But at 450 hp, it’s not blowing its brains out either.

The ISX Varley chose has a torque rating of 1,650 lb ft, which while adequate in this case, could seem a little anemic against a prairie headwind. There, a beefier 1,850 torque rating might be appropriate.

Because of the variations in operating conditions, it’s not unusual for fleets to run several different specs in different applications. This is especially true of post-’02 engines, where engine speed is more critical than ever for fuel economy.

For example, Trent Siemens, maintenance manager at Big Freight Systems in Steinbach, Man., says three years ago his fleet standard rear-axle ratio was 4.33. Today he’s running 3.55s in the tandems and 4.11s in the B-train fleet. They were running Cummins N14s at the time, which wanted to run 1,680 rpm at 62 mph. He has since switched to Cat power, which requires taller gears to achieve a lower engine speed. The Cats like 1,325 rpm for tandem loads, so Siemens had to re-gear the trucks too.

“The engines need to be turning more slowly today, thanks to the emissions rules,” Siemens says. “If you spec outside that engine-speed envelope, it’s going to cost a fortune.”

Is there a perfect spec? Yes, but don’t expect Varley’s spec to necessarily work for you. There’s a lot to consider, not the least of which is getting over the “build-me-what-I-had-last-time” mindset. Mark Wille, sales manager at Mississauga’s Peel Mack, hears that a lot. “Trouble is,” Wille notes, “we don’t make trucks like that any more.”

A lot has happened in truck engineering over the past five years. If you’re on a five-year trade cycle, you’re looking at a whole new drivetrain spec to meet today’s engine operating parameters.

Keep HP, aerodynamics and road speed in mind
when spec’ing for fuel economy


Universally, engine speeds have come down in order to maintain fuel efficiency in the face of the stricter EPA emissions standards. What was once called the “sweet spot” is now a sweet dot. Customers and sales reps have to work closely to achieve the right spec for the intended application. Tire size, rear axle ratios, and transmission selection hang in the balance. Remember that in most cases, it’s torque that makes the engine drivable, not horsepower. In spec’ing the driveline, care should be taken to match roadspeed and engine speed in such a way that drivers have some latitude between optimum running speed and peak torque. Otherwise, they’ll be shifting gears all day, like with the old 318s. Each engine maker can provide gearing recommendations to keep their engines running most efficiently.


There are three schools of thought here — sorry to be so non-committal: wide-step boxes (8, 9, and 10-speeds), multi-speed boxes (13 and
18-speeds), and/or automated boxes.

Fleets have historically spec’d 9 and 10-speed transmissions for a number of reasons: they’re lighter, they cost less, and they’re more driver-proof. When you’re buying hundreds of them at a time, the savings can be substantial. Operationally, the 9- and 10-speeds are adequate for most lightweight applications (80,000 lb or less).

How adequate depends a lot on geography, says Ed Saxman, powertrain product manager, Volvo Trucks. “A linehaul truck on flat ground will be absolutely fine with a wide-step transmission. Fuel economy is always lousy in any gear lower than top gear, so the sooner you get into top gear, the better. And the fastest way to top gear is with fewer shifts,” Saxman notes.

“On the other hand, the multi-speed boxes let you match engine speed more precisely to road speed,” he says. “Take California, with its 55 mph speed limit. If you’re geared to run 65 or 70 and you have to drop back to 55, you’re going to have to run at least one gear down from the top. That will really hurt your fuel economy because there’s a less efficient transmission of power through the gearbox.”

Chuck Blake, special projects manager at Detroit Diesel Corp. says the wide-step transmissions can force an undisciplined driver down into the higher torque range of the engine before making a downshift. “The temptation to split gears on a small grade to keep the speed up can be great,” he says. “You’re better off pulling that hill at a lower engine speed.” Blake also points out that a sharp driver can use a 13- or 18-speed to some advantage by splitting gears to keep the engine closer to its optimum operating speed.

As for automated transmissions, they’re available in a number of configurations (same as manuals in most cases), but they take the driver out of the loop. Varley reports tremendous success with the automated boxes in a few special cases.

“We had a driver who’d make three or four shifts on the Keele St. hill on Highway 401 in Toronto [a modest grade],” he says. “His economy was in the toilet because he was running at the top of the horsepower curve. The automated box makes the hill with one downshift. It was the noise that was getting him over the top, not the torque, and we’ve now solved that problem.”

While the jury is still out on whether or not automated transmissions are more fuel efficient than manuals, it’s safe to say that when programmed properly, they’ll make the shifts at the appropriate points, which is often more than one can say for inexperienced drivers.

Drive axles:

Final drive ratios are coming down. As Wille noted, they don’t make trucks like they used to. Today’s engines need long-legged rears to keep the engine revs down — gear fast, run slow is the new mantra. Which ratio is best is a matter of some calculation. Cat, for example, recommends a C-ratio 10-speed with 3.36:1 gears in a tandem linehaul application mated to its 435-hp C13 ACERT engines. Or, in the same application but with a ZF FreedomLine 16-speed, a rear ratio of 3.07:1. For heavy-haul, Cat suggests a ratio of 4.11:1 with a 550-hp/1,850 lb ft torque C15 and a B-ratio 18-speed. Just don’t expect to get through that discussion over a single cup of coffee.


Here are a few general rules about the spec’ing choices and their effect on
fuel economy.


Spec for torque. High horsepower and high torque aren’t necessarily synonymous: Cummins offers 1,850 lb ft in a 475 ISX, while Cat boasts 1,750 lb ft in a 435 C15. Other engine makers offer 450-475 hp engines in the 1,550 lb ft range — all more than adequate in the right application.


A truck pulling a 48-ft trailer moves about 18 to 20 tons of air per mile. Between 55 and 60 mph, 50 percent of the fuel burned is just to overcome wind resistance. The advantages of spec’ing an aerodynamically efficient tractor cannot be overstated. But consider the application, too.


Speed comes at a price. Period. Kenworth’s extensive fuel economy testing program (see In Gear, page 45) has determined that running at 65 vs. 60 mph yields a decrease in fuel economy of 6.4 percent. And it gets worse the faster you go. Ready for 75 vs. 60? How does a 17.3-percent hit grab you? Whether the truck is governed, or speed restrictions can be achieved through incentives, it’s paramount that roadspeed be kept to a minimum. Gear fast, run slow is the way to go.


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