In recent years, drivers and owners of Class 8 tractors have had a huge selection of horsepower choices for their over-the-road rigs. If you asked some of them, "How much horsepower do you need?" the answer would be, to paraphrase James Dean in Rebel Without a Cause, "How much do you have to offer?" For others, the selection of horsepower is based more on considerations of fuel economy and pulling efficiency over raw power. Like many other aspects of trucking, horsepower decisions often mix business need and personality traits.

Dwain Guyton, now a recruiter for USA Truck Inc. in Van Buren, Ark., spent many years as an over-the-road driver before taking up his current duties. "I never have had a big horsepower truck," Guyton says. "What I look for is torque, the pulling power to get over the big hills.

"Horsepower itself is more about speed. I want to know that my engine can pull up a hill, fully loaded, not let me down and get good fuel mileage, too. Our trucks do that. I find that the money that some companies choose to put into big horsepower, this company puts in my pocket, where it counts to me," Guyton says.

On the other side of the coin, independent truckers like Dana and Brett Cole of Joplin, Mo., have different priorities. Instead of fuel economy, the Coles want performance and style. Their beautiful custom flattop Peterbilt 379, which Brett calls "Simply Radical," has admittedly poor aerodynamics but sure has a lot of style.

Pulling a reefer through the deserts, where fierce headwinds are common and high speeds are legal, requires a lot of horsepower. Cole currently uses a 550-hp Caterpillar engine and says he gets about 5.5 miles to the gallon. "On my next truck, I'll probably get the 600-hp unit. Using a flattop sleeper and pulling a reefer combination robs me of about 150 hp. And that costs a little extra in fuel. But I love the looks of the rig."

Phil Hook Sr., truck operations specialist for Caterpillar Engines, loves a good-looking rig as much as anyone, but says looks should take second place to economy. Hook regularly achieves 7-plus mpg driving his fully loaded 1997 Kenworth T2000 fitted with a 732-cubic-inch C-12 engine rated at 430 hp and 1,650 lb.ft. of torque. To accomplish this, he chooses to limit his speed, but isn't hemmed in by a governor. He generally drives between 58 and 63 mph, at rpms between 1200 and 1575. If he needs more speed, it is available.

"This is a new age. You have to develop new driving styles for the engines today," Hook says. "Using the old style of running 1,800 to 2,100 rpm will cut both your performance and mileage way down, and it's not necessary. The new tools will do the job well, you just have to learn how to use them." Each driver makes valid points. But for most owner/operators, small fleet owners and drivers, the answer to the question of how much horsepower they actually need will lie between two extremes — limiting overall speed on one side and trying to get the best fuel mileage a truck can reasonably deliver on the other.

What you need, when you need it

Gary Graham, an owner/operator turned truck designer, has spent thousands of hours studying the problem of getting maximum fuel efficiency. His highly modified T600 combines unique aerodynamic styling with a 280/350 variable horsepower Detroit Series 60 engine. It gets between 8.5 and 13 mpg, Graham says.

"For basic just going-down-the-road, you need about 280 hp," says Graham, whose estimate is in line with most engine manufacturers' available data. "To get over the big hills, you need between 350 and 400 hp. Only about 3% of our Interstate highway system is comprised of big hills, so you don't need that extra horsepower all the time," he claims (others estimate steep grades make up as much as 10% of U.S. highways).

"Thanks to electronics today, you can program an engine to meet the current conditions effectively," Graham says. "The key is driver training — not preset limitations on the truck. Teach the driver how to get fuel economy, set up the truck right, and let the rig roll with the flow of traffic at legal speeds." The people at Detroit Diesel see a similar picture where horsepower, truck design, speed and driving styles work efficiently together. They note that the 500-hp Detroit Diesel Series 60 can achieve 7.9 mpg at 60 mph using an 18-speed transmission and 3.08 rears. This highly economical performance can be lost easily when the truck is reconfigured for higher speed.

The 550-hp Detroit, using the same 18-speed transmission and 3.55 rears, for better hill climbing ability, is estimated to get 5.9 mpg at 70 mph. That 2-mpg difference with recent fuel prices averaging $1.43 translates into $7,000 a year difference in operating costs for fuel alone.

There was a time when engines had what was known as a "sweet spot" where you got maximum power and fuel efficiency combined. The electronic engine is able to adjust itself to a variety of conditions while in motion. Consequently, a driver does not need to aim directly at a rpm range and keep moving the gears to hit that spot.

A Cat C-12 engine may run at peak efficiency at 1,450 rpm on flat ground with an 80,000-lb. load. But as the truck heads uphill, the rpms may drop to 1,200. Under the old rules, you downshifted to raise the rpms to the "sweet spot" of 1,450 rpm. In practice today, you may actually lose fuel efficiency by downshifting if the engine is able to maintain 1,200 rpm.

The only accurate way to determine if these new "drive-by-wire" engines are reaching their "sweet spots" is to use an electronic monitor like Caterpillar's ID, Detroit's ProDriver or Cummins' Cadec RoadRelay systems. These monitors provide real-time fuel consumption data that prompts the driver on what gear is needed for each situation.

Heavy Lifting

It is important to remember that horsepower is a fixed value. Torque does the heavy work of pulling a rig over the hill. The amount of available torque at the wheels depends on the drivetrain. An engine rated at 1,650 lb.ft. will deliver 5,534 lb.ft. to the ground using a 3.70 axle ratio and a transmission ratio of .86 (approximately 12th gear in an Eaton 13-speed double-over transmission). Drop into 11th and torque rises to 6,105 lb.ft. at the wheels.

Ultimately, it takes a matched engine and drivetrain combination, and a driver who understands the tools at his command, to deliver decent fuel economy and keep shifting to a minimum. The best answer to the question of how much horsepower is enough appears to be, "Are you meeting your needs and if not, which path is best for you in your application?"