Going the Distance
Improving fuel economy on older trucks
By Paul Abelson
There’s no doubt that today’s trucks get far better fuel economy than new ones got just a few years ago. A short time ago, Freightliner and International publicly argued over whether the Cascadia or ProStar had better aerodynamics and, therefore, better mpg. This year at MATS, PACCAR introduced the Kenworth T700 and Peterbilt 587, among the most aerodynamic tractors ever made.
But few of us can afford new tractors, especially in the current economy. Don’t despair. There are many ways to improve fuel economy in older trucks. Some contribute a little, some a great deal, but taken together the results can be dramatic. Imagine going from 5.5 mpg to 6.5 mpg, an 18 percent increase. With $3.00 diesel, that could add almost $8,400 to your bottom line every 100,000 miles you drive. That can pay for quite a few devices. Many offer payback within one year.
These aren’t “magic bullet” devices, things that you put in your tank or splice onto your fuel line, that promise to increase mileage 25 percent, cut emissions 50 percent and keep your hair from falling out. The devices I refer to have been tested according to SAE J1321 and TMC RP1102 standard fuel economy test procedures. Most have been accepted by the EPA as part of the SmartWay program. In other words, they work!
Rolling down the road
Driving practices account for as much as 35 percent fuel economy variations between the best and worst achievers. Avoid jackrabbit starts, practice progressive shifting, don’t brake hard or drive faster than necessary and use equipment already on your truck.
Almost all trucks have cruise control, but not that many drivers use it to minimize fuel use. The computer knows the best amount of throttle to use to hold — and to reach — set speeds. When accelerating back to cruise speed, let the “resume” function manage fuel delivery.
In the past decade, the vast majority of trucks came with Jake Brakes or other engine brakes. They sound so cool (to some drivers) when they’re on, but every time they are, they waste fuel that you used to get to speed. Think of that pop-pop-pop as fuel or dollar bills going out your stacks. To minimize wasting fuel, set the Jake to come on at about 10 mph below your cruise set speed, leaving a margin to coast down without the brake applying.
Tires play a huge part in fuel economy. As they wear, you actually gain mpg since tire rolling resistance diminishes. A tire half-worn gets about 4.5 percent better mpg than a new one. At 80 percent that becomes 6.5 percent, so don’t replace tires prematurely. Tread is a major contributor to rolling resistance. Compound and tread design significantly affect efficiency. Rib patterns are far more fuel efficient than lug designs, by as much as 4 to 9 percent. Look for SmartWay-rated fuel-efficient tires.
Wide base, single tires save fuel because they have half as many sidewalls to flex, which absorbs energy. With fewer sidewalls and a stiffer tread, less energy is needed. Wide base singles save weight, about 70 pounds per dual. Less weight means more revenue or less fuel used. Replace eight sets of duals and you’ll save 0.7 percent fuel used for weight alone.
Air pressure supports the truck and affects flex. Underinflation of 20 percent (80 psi vs. 100 psi ) reduces mpg by one percent.
Engine accessories affect economy. Modern cooling fans consume more than 50 hp when on. With on-off fan clutches, make sure your fan-on and -off settings are properly adjusted. If you can cut fan time from 50 percent to 20 percent, fuel economy improves by 9 percent.
Evans Cooling Systems makes a waterless coolant. With no water in the coolant, system pressure can be reduced from 15 -18 psi to only 2 psi. The coolant operates at higher temperatures (get a 215 degree thermostat) where diesels are more efficient. In SAE/TMC tests run by Auburn University, a test truck got more than 3 percent better mpg.
Maintenance is critical to keeping fuel economy where it was designed to be. Run the overhead at least annually. Excessive valve lash can rob power, so you burn more fuel. Test your charge air cooler. Leaks cause the engine to run rich. Always use good quality diesel. Poor fuel can clog injectors, altering spray patterns and reducing combustion efficiency. Check fuel pump output and injector O-rings.
Restriction at the air cleaner lowers fuel economy. A new one will provide a 1 percent improvement compared to one with 25 inches of restriction.
Consider switching drive train fluids to synthetics. Savings can range from 0.5 to 2.0 percent, compared with standard mineral oils. And something as basic as cleanliness is being studied for its effects on fuel economy. Dirt affects air flow over surfaces, and reducing interference reduces fuel use. Keep your truck clean and polished and you’ll not only improve its appearance, you’ll help laminar air flow.
The savings presented here are small compared to the double-digit improvements achieved when tractor aerodynamics were introduced in the early 1980s. Aerodynamics is once again emerging as a source of fuel savings, but now at the trailer. Early thinking was that the trailer was just a box you put your load in, pulled by a tractor that cut the air. Trailer aerodynamics didn’t much matter. But research has demonstrated that while current tractor-trailers have a coefficient of drag (Cd) of 0.60, improvements to trailers can reduce that to 0.35. That can mean almost 15 percent higher mpg.
New aerodynamic devices are now available and SmartWay-approved to help you enter California CARB-compliant. ATDynamics TrailerTails are simulated boat tails. They offer more than 5 percent improvement. Various trailer skirts that keep air away from turbulent under-trailer and tandem areas improve air flow and offer another 5 percent or more improvement. Other trailer devices generate or manage vortexes at the front of a trailer in the gap and at the rear.
Add a device here, improve a practice there, and soon the total savings become significant. Not only do these devices improve fuel mileage, they pay for themselves in 1 to 2.5 years. Then it’s all money in your pocket.