Bottom Line
Improving the Economy
Driving smarter

by Paul Abelson

There's no need to tell anyone reading Land Line that fuel costs have climbed dramatically, and that any change in what is already the highest cost item in trucking (other than personnel costs) will have an equally dramatic effect on annual profits. So perhaps now is the best time to make those improvements to get the greatest return on your fuel dollar.

To put things in perspective, let's review some basics about diesel fuel, how it works, and how you can best manage its use. We'll start with the fuel itself. Diesel is a mixture of hydrocarbons (see the article on additives in this issue) that can be looked at as stored energy waiting to be released. Diesel, which is refined from crude oil, is the result of the decomposition of organic matter, liquefied under tremendous pressure over millions of years. As a refined fuel, it has the potential energy of about 135,000 British Thermal Units (BTUs) per gallon. A BTU is the energy it takes to heat one pound of water one degree F. The values are approximate because diesel can vary in its makeup.

Not all the energy is captured to do useful work when diesel is burned. First of all, not all the fuel is completely combusted. If it were, we'd have only carbon dioxide (CO2) and water vapor (H2O) in the exhaust; no soot and no hydrocarbons or particulates. Second, there are heat losses into the cooling system and out the exhaust. Some of the exhaust heat is captured and reused in the turbocharger, but in a modern engine, barely 40 percent of the energy in the fuel is harnessed to do useful work. In the '80s, this was only about 33 percent, so engines have made significant progress in the last 15 years or so.

The important thing to remember is that every bit of energy your truck uses comes from just one place: the fuel in your tanks. Unless you're hooked up to 110V shore power, diesel provides all your energy needs. Even generator sets or fuel-fired heaters run on the same diesel your engine uses. And anything that uses more energy than is absolutely necessary means that you burn more diesel. Let's look at driving habits as they relate to energy.

Rolling resistance

Energy is consumed by overcoming aerodynamic drag, internal friction, rolling resistance and by accelerating and decelerating (braking). Internal rolling resistance is largely a function of truck design, and other than using low-friction lubricants, there isn't much you can do to manage it. Rolling resistance is a mix of controllable and non-controllable factors. Tread design and tire carcass construction play a large role in rolling resistance, and they, too, are outside a driver's control. But air pressure is within your control, and you can check alignment.

Tires flex when they roll and flexing absorbs energy. Air is a structural part of a tire. The more air that is in the tire, the stiffer it is and the less it will flex. Tires 20 psi low on air can lose more than 3 percent fuel economy. Tire alignment is important because if a tire is not rolling absolutely straight, it is being dragged sideways, and rubber is being rubbed off the tire. It takes energy to do that. The more the tires are out of line, the more energy is wasted.


Aerodynamic drag is a function of three things, two of which are outside your control. Frontal area, coefficient of drag and speed make up total drag. The shape of the truck dictates coefficient of friction. Once you buy your truck, you're stuck with your choice, for better or worse. The differences between long-nosed, classic conventionals and the new, aerodynamic trucks could make more than a one mpg difference, all other things being equal. Frontal area is also outside your control, unless you're loading flatbeds (but that's a subject for another time). We'll assume frontal area, the size of the hole your truck punches through the air, to be constant. But closely related to frontal area, the gap between tractor and trailer can affect aerodynamic drag. The closer the two are together, the less the air will fill in behind the tractor and impact the trailer's front wall. Spread them out more than 36 inches and it starts to be like pulling a second front through the air.

The greatest variable is speed, because aerodynamic drag varies, not directly with speed, but with the square of the speed. Sixty-five miles an hour is only 18 percent faster than 55, but drag increases by 40 percent. Controlled tests run by TMC demonstrated that increasing maximum speed from 55 to 65 added between five and eight miles an hour to average speed, depending on terrain and traffic, but took as much as 18 percent more fuel. So holding speed down is important. I'm not advocating going back to the days of 55 mph limits. Far from it. But if you know you'll have time to make your delivery, why push the speed limit? There are reasons the big, profitable fleets limit their trucks to 65 or 70. It saves fuel, and it's easier on equipment. If you have time to drive 68 instead of 75, do it. The difference in aerodynamic drag is 21 percent, and you'll save more than 10 percent in fuel mileage.

Potential energy savers

Acceleration takes energy. Driving a truck isn't drag racing, so you don't need to be the first to get up to speed. Acceleration (the rate at which speed changes) is a significant contributor to energy consumption. The faster you accelerate, the more fuel you burn. It's that simple.

Progressive shifting is being taught in drivers' schools these days, and progressive fleets are teaching it to their drivers. Many savvy owner-operators already do it. Instead of running up to red line in every gear, progressive shifting lets you take advantage of torque multiplication through the gears. The lower the gear, the greater torque is multiplied, so you don't need to run the engine up as much. Remember, just turning the engine over, moving the pistons up and down and spinning the crankshaft, take energy. The fewer revolutions the engine has to turn, the less fuel it will burn.

Idling of the biggest killers of fuel mileage

 There are times when you'll absolutely have to idle, when the temperature drops way down, and you don't have a heater, or when your accessories have run your batteries down and you need to recharge them. But how many times do we see trucks idling on a beautiful evening, with the temperature somewhere between 65 and 70. The drivers sure don't need air conditioning, and they shouldn't have any problems starting. Yet they idle, probably out of habit. Let's see what idling does to fuel mileage. Assume a driver runs 13 hours in a 24-hour period, covering 650 miles. That's an average of 50 mph - a reasonable speed. To keep the math simple, he gets 6.5 mpg, so he uses 100 gallons on the road. But for the remaining 11 hours, he idles, using 1.25 gallons of fuel an hour. That day, he burns 113.75 gallons, so his overall mileage drops from 6.5 mpg to 5.7 mpg. That's almost a 14 percent drop. Controlling idling saves wear and tear on the engine, and it also saves money. At the recent national average price of $1.46 per gallon, the driver in our example would save $20 a day. Driving 250 days a year, that could total $5,000.

Braking wastes a great deal of energy. If that seems like a contradiction, consider that when the brakes reduce your truck's momentum, its kinetic energy (by converting it into heat) the energy used to hold the truck at speed is being absorbed and cast away as waste. Fuel was burned to bring you up to speed and to hold you there, so fuel is what is wasted. You can reduce the amount of energy lost as heat by anticipating stops and coasting to exits. Jim Booth, former trainer-driver for Caterpillar, would often coast in top gear for a mile or more approaching his exit. Climbing hills, he would ease-off approaching to top, and let his momentum carry him over the top. He let gravity work for him, not against him. Cresting hills slowly, he could more easily get into the right gear to take him safely down the next grade. He'd barely touch his brake on the way down.

If you have cruise control, use it. The engine computers are set to keep you at a smooth, steady speed. Even when climbing hills, stay in gear and in cruise as long as you can. Don't try to help the engine by tromping the pedal. Overall, you won't lose much time. Jim used to tell about the drivers he'd see passing him on the highway. They'd speed to where they were going, but often be more tired than he. Whether it was to stop for fuel, for personal needs or to rest, Jim couldn't know, but cruising at or under the speed limit, he'd see the same trucks pass him as much as three or four times a day.

Next issue, we'll look at fuel saving equipment, and how to specify and maintain trucks for maximum fuel and operating economy. To sum up, just drive as if there are raw eggs between your feet and the pedals, and there's a bonus if you get to your destination with the eggs unbroken. Your gentle touch will assure the eggs stay safe. There is such a bonus. It's your fuel savings.

Paul Abelson is Land Line's technical editor and freelances from his office in Lisle, IL.

Aug/Sept Digital Edition