What Drives Truck Specs?

Engines: Electronic engines burn less fuel that mechanically controlled engines because they provide more precise fuel injection timing and higher cylinder pressures. Engine manufactures have different recommended cruise revolutions per minute (rpm) settings for different engines.

Spec the appropriate size engine for the job. An electronic engine improves mileage (miles per gallon - mpg) by 7 to 15 percent. Also, operating the engine at 100 rpm at cruise miles per hour above the recommended rpm can improve mpg by up to 3 percent. Cruise control vs. no cruise control improves mpg by up to 6 percent, according to TMC.

Radiator Shutters: Shutters on over-the-road (OTR) tractors remain closed 60 to 70 percent of the time — even in summer conditions — and improve fuel economy through more precise engine temperature control and improved vehicle aerodynamics. Engine efficiency increases with radiator shutter activation set 10 to 15 degrees F above thermostat settings. Using shutters in the summer could improve mpg by 0.5 to 2 percent and by 1 to 3 percent in the winter.

Intake and Exhaust Restriction: Fuel consumption decreases as the effort required to move air into and exhaust gas out of an engine increases. With no intake restriction vs. 25 inches of water restriction, mpg improves up to 1 percent. No exhaust restriction vs. 40 inches of water provides a 0.33 to 2 percent improvement.

Cooling Fans: Viscous fan drives are energy wasters; they may operate 10 to 30 percent more than necessary, pulling as much as 50 horsepower from the engine and causing premature fan-clutch or control system failures. Consequently, spec the smallest variable speed fan possible that still serves the function. Using an on/off fan instead of a viscous fan can improve mpg by 1.5 to 5 percent.

Air Compressors: Consider how large an air compressor the vehicle requires. Large displacement compressors exact a fuel consumption penalty. If a 12 to 13 cubic feet per minute (cfm) compressor is used instead of a 15 cfm to 17 cfm compressor, mpg can improve up to 0.5 percent.

Idle Limiting Devices: Idling engines cost money by increased fuel consumption and additional maintenance, but it's difficult to quantify exactly how much. Nevertheless, test show that class 7 and 8 heavy duty vehicle diesel engines consume 1 gallon of fuel for every 2 hours of idling. With the engine and air conditioning on, zero idling instead of a 25 percent rate will improve mpg by 3 to 6 percent.

Tires: New tire construction differs significantly among manufacturers — even among a single manufacturer's product line. Typically, the amount of tread remaining indicates mileage performance. Fuel consumption figures also are affected by tire design, construction, wear and vehicle placement. The tire type could affect mpg (mileage) by up to 14 percent.

Transmissions: Transmission gearing for the best fuel economy depends on engine design, routes, driver preferences, truck performance and more. Up to 4 percent savings can be realized by using direct-drive transmissions in the highest gear vs. overdrive transmissions.

Axle and Transmission Lubricants: Synthetic lubricants outperform mineral lubricants at all temperatures; the lower the temperature, the greater the improvement. Synthetic lubricants are said to improve fuel economy by 2 percent in the winter and by 0.5 percent in the summer.

Single Drive Axles: Tandem drive tractors have an interaxle differential and a second axle ring and pinion. This increases weight, friction and power requirements, and more fuel is consumed. However, there are clear fuel savings from tag axles. Users should evaluate potential operational problems, such as possible increased tire wear, traction loss in bad weather and resale value loss — before spec'ing this drive setup. Using a single drive with a tag axle will improve mpg 2 percent to 3 percent vs. a tandem drive.

Cab Extenders: Cab extenders, or cab-mounted side farings, are aerodynamic devices that reduce trailer gap, the open space between the back of the cab and the trailer front. Reducing trailer gap reduces air turbulence, or drag, around the truck. A truck equipped with cab extenders but without a cab-mounted deflector or roof fairing will increase drag unless the trailer gap is large. Using a 15-inch cab extender can improve mpg by 1 to 4 percent.

Air Deflectors: Most air flow devices are effective in reducing fuel use, depending on the application. A 4 percent to 10 percent loss in fuel economy is caused by air directed into the top front of the trailer, rather than over the top and along the sides, as is the case with a full cab-mounted fairing package.

Break-In Periods: No changes should be made based on fuel consumption figures until trucks have been used for 10,000 miles. Also, tires should have about 50 percent wear because the less tread remaining on a tire, the less road resistance and less fuel consumed. A truck with 10,000 miles vs. none will show a mileage improvement of 2 percent to 5 percent.

Lightweight Components: The lower the weight of the vehicle, the less fuel required to move it. But, lightweight components often cost more and sometimes are easily damaged, so weigh the potential value vs. cost before buying.

Seeing the Difference

Spec'ing for safety has become an increasing concern because of new government regulations. Brakes certainly are an important part in providing accident-free driving, and the use of anti-lock braking systems (ABS) and electronic braking systems (EBS) are improving this system.

However, vision is key to knowing when to brake. TMC research shows that 90 percent of all driving decisions are based on vision alone. Consequently, windshields, wipers, all lights (but especially properly placed headlights), and mirrors are increasingly important items to specify.

Whether fleet managers are spec'ing for safety, resale value, easy maintenance, fuel economy or another objective, remember to analyze the options and choose wisely. Because in the end, original specifications are paramount for maintaining and valuing vehicles.

Bob Deierlein is Waste Age's truck editor

PRIORITIZING OPTIONS

Fleets managers generally set truck specs to meet operational requirements. However, some specifications provide a particular objective. For instance, sometimes you must spec for fuel economy, driver satisfaction, resale value, or safety.

It's important to prioritize operational needs — minor tradeoffs may be necessary to reach the highest priorities. To help, The Maintenance Council (TMC) of the American Trucking Associations, Alexandria, Va., recommends the following tips.

Body Installation: Be sure the chassis provides for the easy installation of the body. For example, the chassis must have clean frame rails and junction boxes that are easy to tap into for body and equipment installations to eliminate splices in lines.

Brake valves and other components that need future replacement also should be easily accessible once the body is installed. Some grease fittings must be turned for access, and air valves should be accessible. Batteries and cables should be secure and routed separately from the battery box, if possible.

Air lines and wiring harnesses should be routed so that they are secure but aren't rubbing against other components. They should have protective coverings, if necessary.

Driver Satisfaction: TMC studies show that a happy driver is more productive. Some of the items refuse drivers consider important for comfort as well as performance include:

Automatic transmissions;

Air conditioning;

Radio;

Easy turning and handling;

Good visibility; and

Double air-ride seats.

Fortunately, newer trucks are easier to operate because features such as seats, suspensions, etc., are air-controlled, which helps to eliminate swaying and provides a safer truck.

Maintenance: When spec'ing for a low-maintenance and low-cost operation, fleet managers should pay attention to front-end items: springs, spring pins, bushings and wheel-end assemblies. The five main areas to consider, in order of importance, are tires, brakes, wheels, suspensions and seals. — Bob Deierlein