Unlike fleets, owner-operators get one chance to spec their equipment correctly,” says Charles Allen, director of sales and marketing for ZF Meritor. “These days, owner-operators could do worse than follow fleet specifications for engines and transmissions.”

To understand why, it helps to grasp the relationships among torque, horsepower, gearing, rear axle ratio, axle weight ratings and tire load rating. While big power and big torque are seductive, choosing such options without considering their roles in your operation may result in plenty of ego satisfaction but also lost profit due to unnecessary costs in purchase price, parts, maintenance and fuel economy. The owner-operator who carefully matches transmission with engine can end up with a cost-efficient truck suitable for his application.


Engine performance has changed dramatically in 10 years. Horsepower has migrated upward, much to the delight of owner-operators. With older engines, torque came on suddenly and dropped off suddenly, making it difficult to maintain good operating performance. “Newer engines have flatter torque curves,” says Mark Conover, a Cummins marketing strategist. Flat torque means that pulling power is available through a broad rpm band, generally 1,100 rpm to 1,500 rpm.

“Contemporary engines have much wider operating ranges and almost constant horsepower throughout the operating range,” Allen says. For those applications that need the close-spaced steps of yesterday’s 13-, 15-, or 18-speed transmission for reasons of power, acceleration or fuel economy, he says, today’s 10-speed “may actually be perfectly suited, without the cost premium or operational complexity.”

Owner-operators are unlikely to spec any horsepower below 450 today. A horsepower rating that high, with a midrange (about 1,550 pounds-feet) torque rating, is sufficient to run coast to coast. What determines the efficiency of that torque and horsepower is gearing and rear axle gear ratio.


Smaller, closer-stepped gears will provide more torque and, therefore, more pulling power on hills. Much depends on the rear axle ratio – a measure of the torque that reaches the ground. A rear axle ratio of 3.90, for example, is suited more for power than speed. A tall rear axle ratio, say 3.08, will give speed on the flat. For road operations, these ratios are somewhere near the top and bottom of practical application.

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However, gearing is not the only thing to consider. “The number of engine, transmission, drive shaft, axle tire combinations can be overwhelming,” says Al Zwicky, senior applications engineer for Peterbilt. “Peterbilt’s electronic spec’ing tool, Prospector, gives a buyer the opportunity to input the performance he wants at a computer with his salesman and watch as the specs are formulated.”

One variable used in the spec’ing process is startability – the amount of torque it takes to get a truck moving. Given a specified torque rating, where your gearing steps start will determine how much startability you have. If you haul coal in the Keystone, you will want a lot of startability. If you deliver in urban areas and run the big road, you can probably get by with low startability and still start in second on a hill with 40,000 pounds in your wagon. Road tractors are generally geared to have plenty of startability for the typical road application.

You can also enter gradability into a spec’ing program. This is based on speed, grade and weight. For example: What specs are needed to go 65 mph on a 3 percent grade when loaded to 75,000 pounds?

Tom Reder, an owner-operator with Munster, Ind.-based Area Transportation, hauls steel with his 2000 Pete 379. Using Prospector, he spec’ed for heavy loads and a variety of terrain, opting to maintain gradability with short gear ratios. Reder’s Pete has a Cummins N14 rated at 525 hp, 1,850 pounds-feet of torque, and 3.73 rear axle ratio.

“I like the short gears with the 18-speed and the double-over for Pennsylvania,” Reder says. “I can split when I want to, and there’s plenty of grunt for the mountains.” Reder also has a truck running construction sites, into which he has put 3.90 rears that give “startability when I need it off the pavement.”

The other side of the equation is cruise speed and overall ratio. Higher cruise speeds will take away from the vehicle’s pulling power on hills. Tall top-gear ratios give speed, but mean more shifting on hills to maintain gradability.

Owner-operators traditionally want big power and 15- or 18-speed transmissions, despite the potential savings of optimizing spec’ing for fuel mileage. “A buyer who wants big power can spec his truck far too fast to be practical,” says Ed Saxman, director of power train systems for Volvo Power Train Corp. “Cruise speed should take speed limits and the need for power to climb into consideration.”

The higher horsepower ratings popular with big transmissions make less sense with lower torque ratings. There is a generally understood range of horsepower for a given torque rating.

“Higher torque is necessary when a vehicle is towing a large load or working in rugged conditions,” Allen says. The more gears a transmission has, the more precisely and efficiently it manages torque – the transfer of power from the engine to the transmission – and horsepower. Speed changes in an 18-speed are generally smaller than in a 10-speed, for example.

And while big power and more gears seem to go together, Saxman notes that a 500-hp or 600-hp engine with a 10-speed will perform just fine. On hills, drivers with 10 speeds can use the modern flat-torqued engines to pull a little slower because there is less overall ratio coverage. Using the entire rpm range, in particular the bottom end between 1,100 and 1,300, where the engine can now safely lug, saves fuel. Fleets spec wider steps for this cost savings. The owner-operator, having spec’ed smaller steps, may feel he compensates for lost fuel efficiency by getting where he’s going faster or simply by adding to his job satisfaction.

Highlighting buyers’ needs to spec for their particular operations, Conover at Cummins concurs with Allen that every vocational niche has its gear box. “There is a weight and cost savings with 10 speeds for highway applications when not a lot of torque is needed,” he says. “But in some applications, like grain hauling and others where the truck will be in soft terrain, closer-stepped transmissions help a driver stay on top of the power curve.”

The closer-stepped transmissions have an advantage on the big road too, according to Saxman, because the experienced driver knows when to “dial them in.” For example, rather than cruise in ninth at 55 to obey the California speed limit and have few rpm left at the top of the gear, or be in 10th and lug, a 13- or 18- can be split to pick the desired rpm.

Close-stepped trannies are certainly desirable when climbing, also. Then, too, all those split gears need not be used all the time. As Conover says, “Wider-stepped trannies run in efficient operating range, between 1,100 and 1,300 rpm. Drivers can let the direct shifts lug. The driver with the 13- or 18- can simply not split and achieve the same effect.”

Buyers must consider their personal driving habits, especially what part shifting plays. You may decide to go for the 18 because you want to chase horsepower and dial in your performance. You can’t do that with a 10-speed, even with today’s flat-torque engines. On the other hand, you may decide to split gears only in certain situations and let the torque flow through the tranny unhindered by the extra gearing required by splitting. Leave it in direct and your tall gears will get you good fuel mileage, and you will have one mother of a pulling truck to boot, even with the big ratios.


Adding to the complexity of choices is a variety of drive shafts rated between 1,350 pounds-feet and 2,050 pounds-feet. Engine manufacturers often stipulate drive shaft and axle ratings based on the vocational niche of equipment and horsepower ratings.

Cost rises, as does weight, when torque ratings go up. Deuce Kauffman, division applications manager for Peterbilt, notes, “There can be 500 pounds or more difference between the lowest and highest torque-rated shafts and axles.” Zwicky notes that axles have weight ratings as well as torque specifications.

Because tires are spec’ed to go with the weight rating of their axles, spec’ing isn’t finished until the tire is considered. Some owner-operators use low-profile tires to get more tire revolutions and return some pull to the wheels, especially if they have a tall rear axle ratio. On the other hand, many owner-operators spec tall rubber for ride, better wear and better tire resale than the low-pros.

Optimizing drivetrain combinations can be complex. However, the advent of electronic spec’ing is making it much easier to select components based on application and driver preferences. “Spec’ing will be simpler, but choices will be better,” says Mark Vehlewald, manager of International’s Heavy Vehicle Center Marketing. “It will be more difficult to order a bad truck. Every truck will get optimum spec for the truck’s application.”

Besides having torque ratings, axles have weight ratings to consider. Make sure you spec for both. Above: Arvin Meritor’s MT-40-143-MA-N tandem axle.


Consider Earl Evans’ Kenworth W900. Evans, an owner-operator now pulling air freight between Pittsburgh and Cincinnati, has geared very tall, at 3.08 rear axle ratio. He runs a 600 Cat with 2,050 pounds-feet of torque and uses an Eaton Fuller 18 with two overdrives. He could do 120 mph with that gearing. From Evans’ point of view, it’s not a waste of potential energy, but the best of both worlds.

While Evans’ rear axle ratio preference runs contrary to the popular wisdom that would call for shorter gears, it works for him because of his well-honed driving skills and knowledge. He realizes his spec’ing would not be appropriate for everyone and notes that manufacturers hesitate to spec trucks as he has spec’ed his.

Because he does not split gears all the time, he says, “I get all the horsepower through the transmission without turning all those gears.” It helps that he drives his truck gently. “I don’t crash gears, and I never get above 1,500 rpm,” he says. “I don’t have to. My fuel mileage is a constant 6.4 mpg, and I have all the speed and power I can use.”

Evans, who often hauls heavy and sometimes pulls a spread-axle flat, uses the lower splits to turn corners, especially going up hill. He saves a lot of stress on his front trailer axle and tires by splitting and managing speed and power more exactly.

Evans says he very seldom uses the two overdrives. “They run hot,” he says. “Besides, I can lope in 16th at 1,450 rpm and do 65 on the flat. I’m not doing much more than idling. Red line is 2,100.”

Some buyers might opt for tall rubber to improve ride and maintain resale value, but Evans spec’ed purely for performance, so he chose 22.5 low-profiles. “I wanted to put more tire revolutions to the ground and get back a little torque,” he says.

These specs combined give Evans a truck with excellent performance and good fuel economy – a premium truck with high resale that rides like a Cadillac.