Speed: the biggest factor Load: the second biggest factor

The energy that moves a truck comes from the consumption of fuel. The forces that must be overcome include both aerodynamic and tire rolling resistances, each of which increases with speed. Speed is the largest single factor in large truck fuel economy.

Any form of energy produced or used by a truck must come from the fuel in the tank. That’s basic science.

The heat of the engine, the headlights, the air conditioning, the instrument lights, the truck’s motion, even the sound of the truck roaring by, are all the result of diesel fuel being converted into energy.

Some things influence fuel consumption more than others. We’ll try to take them in order, from the largest to the smallest.

Speed: the biggest factor

Moving a big truck down the road requires the engine, drivetrain and tires to push against several different resistances.

The chart shows the horsepower required to maintain a variety of speeds. The two lower curves show the contribution of air resistance and rolling resistance to the total.

Increasing speed from 55 to 75 mph can increase fuel consumption by over 50 per-cent, while cutting tire fuel efficiency by nearly half.

Air resistance

Air resistance is practically nonexistent at very low speeds but increases rapidly with speed, becoming a major contributor to horsepower requirements—and therefore, to fuel consumption. In fact, once speeds exceed about 45 mph, air resistance is more important than tire rolling resistance.

The shape of the curve on the chart—and how fast it rises—depends on many factors, including the aerodynamics of the tractor and trailer (or trailers).

It’s easy to see that air resistance is a major part of fuel consumption; and why truck manufacturers work so hard to improve the aerodynamics of their equipment.

Tire rolling resistance

Tire rolling resistance is the amount of drag created by the tires as the vehicle runs down the highway. Anybody who has rolled a truck tire across a service bay knows that it takes some effort. But try to do it at 55 miles per hour with several thousand pounds of load on it!

Even though rolling resistance doesn’t increase as fast as air resistance with an increase in speed, the chart shows that rolling resistance is present—and a major factor—at much lower speeds.

Just as with air resistance, the actual amount of rolling resistance and the shape of its curve is influenced by many factors, including load, speed, inflation pressure, tread pattern, tire design and
construction.

Like air resistance, rolling resistance is an important factor that increases with speed. In fact, part of tire rolling resistance is the aerodynamic resistance of the tire itself as it moves.

Because tire rolling resistance is not the only factor involved,
an improvement in rolling resistance doesn’t produce an equal improvement in fuel consumption. Just as in the real world, if only part of your revenue comes from hauling groceries, a 10 percent increase in your grocery freight will not produce a 10 percent increase in your overall revenue.

As truck speed increases, both tire rolling resistance and air resistance increase, but air resistance increases much more rapidly, making it more important than tire rolling resistance at speeds above 45 mph.

Speed and travel time

Bridgestone research shows that speed is the largest single factor affecting fuel economy. In one test, vehicles went from
about 4.5 miles per gallon at 65 mph to about 6.5 miles per gallon at 30 mph.

While 6.5 mpg at 30 mph is an improvement in fuel efficiency of 44 percent, cutting speed to 30 mph is not a very realistic way to save fuel.

A change from 65 mph to 55 mph is a little more practical. At 65 mph, test vehicles achieved about 4.5 mpg. At 55, the figure was 5.5 mpg. That’s about a 22 percent improvement in miles per gallon, for a cost of about 18 percent in extra travel time—half the gain in fuel economy for a much smaller investment in extra time on the road.

As a rule of thumb, Bridgestone tests indicate that for every one mph you increase speed (over 55 mph), you cut miles per gallon by about two percent.

As speed increases, travel time decreases, but so does fuel efficiency. At speeds in excess of 60 mph, the fuel economy loss is greater than the travel time savings. Tire fuel efficiency, especially with fuel-saving tires, is cut nearly in half when speed is increased from 55 mph to 75 mph. Higher highway speeds also increase engine and tire wear. Some tires may not have adequate load capacity at speeds above 70 mph.

Running at today’s higher speed limits

Loss of tire fuel efficiency

With all tires, fuel economy is reduced when speed is increased. But with many fuel-efficient tires, some 45 percent of the fuel efficiency of the tire may be lost when a tire is run at 75 mph instead of 55 mph.

In fact, with conventional tires, mpg drops only about 40 percent when speed increases from 55 to 75 mph, suggesting that fuel-efficient tires may suffer more loss at higher speeds than conventional radials.

At higher speeds, overall vehicle aerodynamics become a much larger factor, reducing the contribution of tires to the fuel economy picture. You might say that a fuel-efficient tire has “more to lose” at higher speeds than a conventional design, which may account for the difference.

Even if there is a significant difference between the fuel efficiency of tires at 55 mph, when speed is increased to 75 mph, nearly half the fuel economy advantage may be lost.

Increased engine wear

And tires aren’t the only things that suffer. Engine manufacturers estimate that maintenance costs may be as much as 10–15 percent higher at 75 mph than at 55 mph. Engine durability could also drop 10–15 percent.

At speeds of 75 mph, some manufacturers require increases in inflation pressure and decreases in load carrying capacity. Consult your tire supplier for information about the tires you use.

Limited tire choices

Worse than that, running at 75 mph could limit your steer axle tire choices. Depending on the manufacturer, you may find that in order to run at 75 mph, you may have to increase inflation pressures and/ or decrease loads. Some tires may not be rated to handle a typical 12,000 pound steer axle load at 75 mph. The result may be fewer tire options available to you.

And, with many manufacturer’s tires, you must increase inflation pressure by five PSI if you’re going to run at 75 mph.

Most Bridgestone radials are rated for use at 75 mph without the need to adjust inflation pressures or loads, and maximum usable sustained speeds are listed in the current Bridgestone Medium and Light Truck Tire Price and Data Book.

With some manufacturers’ tires, you may have to decrease loads and/or increase inflation pressures at speeds above 65 mph. After speed, load has the second-largest effect on fuel economy. Therefore, maximizing payload while minimizing nonessential load is crucial. Low profile radials save weight, cost and FET by comparison to standard profile radials. Low profile radials can also lower vehicle center of gravity for better handling and stability.

Shorter tire life

Finally, it’s well known that treads wear faster at higher speeds. Tests show that removal mileages may be cut 10–30 percent. Irregular wear is more likely because of changes in footprint shape. Tires run hotter, which can reduce casing life and retreadability. And, impact damage is also more likely at higher speeds.

Although some of these things may seem to have little to do with fuel economy, they can have a lot to do with increased operating costs.

As we said earlier, a fuel efficiency component that costs more than it saves is not a good bargain.

Load: the second biggest factor

Bridgestone tests indicate that after speed, load is the second most important factor in the fuel consumption of heavy
duty trucks.

Bridgestone tests indicate that reducing payload by 10,000 pounds produces about 4.4 percent savings in fuel.

Load and fuel savings

Although tests show that reducing load by 10,000 pounds could cut fuel consumption by about 4.4 percent, maximum payload is usually your primary goal.

And there are ways to increase payload—by decreasing non-paying load. Lighter weight accessories and tires (like low profile or wide base radials) can help increase revenue-producing capacity without adding to gross weight.

Low profile tires

Low profile tires weigh less than conventional designs and have the added benefit of reducing trailer height, allowing taller trailers for bulky loads. Center of gravity is lower too, which can result in greater lateral stability, especially in turns. And, lower tire weight means lower Federal Excise Tax (FET), a savings on your initial tire investment.

And contrary to popular belief, Bridgestone tests show that when other factors are equal, low profile tires do not always produce better fuel economy than conventional designs. The primary benefits are in reduced weight, better handling and lower overall cost.

Bridgestone engineers also tested 70-series “super-low profile” radials, finding them less fuel-efficient than regular 75-series low profile radials. This was also contrary to popular belief.

Probably, because 70-series tires are physically smaller than 75-series radials, they must work harder at a given load and speed, resulting in less fuel efficiency.

In tests conducted by Bridgestone, low profile radials provided better fual economy than super-low profile radials, and wide base radials were the most fuel-efficent of all, especially when used on both drive and trailer positions.

“Gear-Down” effect with low profile tires

Because of their smaller diameter, low profile tires in drive positions can have the effect of “gearing-down” the engine.

Low profile tires generally cost less, provide greater lateral stability, and because they reduce both weight and height, can allow increased payload.

Before retrofitting drive axles with low profile radials, check carefully to see whether this gear-down effect will force engine rpms into a non ideal range, because this could actually increase fuel consumption, negating the benefits of increased payload. It will also be necessary to recalibrate speedometers, since smaller tires produce higher readings at a given rpm.

And, if you’re spec’ing a vehicle with low profile or super-low profile radials, carefully consider the effect this may have on engine, transmission and rear axle ratio.

Wide base radials

Another way to save weight is to specify wide base radials instead of dual assemblies.

Besides weighing less than duals, wide base radials generally cost less and have lower rolling resistance than dual assemblies, which can result in a 3–7 percent (or more) improvement in fuel economy.

Wide base radials offer similar but even larger benefits than low profile radials, providing 3–7 percent better fuel economy than dual assemblies.

In tests, wide base radials also prove to be more fuel-efficient than both low profile and super-low profile radials. Like low profile radials, they save weight for increased payload, offer lower cost, better fuel economy and lower FET when compared to the two tires they replace.

In addition, in some cases, especially with certain tankers and hopper-bottom trailers, using wide base radials may allow spring shackles to be moved farther apart, so that the container may be positioned closer to the ground.

This, plus the fact that the tread centerline of wide base radials is typically farther from the vehicle centerline than is the centerline between duals, gives the vehicle a wider “track.” The overall effect is a lower center of gravity, which can produce superior lateral stability.

Some fleets also like wide base radials because they simplify their parts inventories, requiring fewer wheels, lug nuts, etc.

Wide base radials are also inherently more fuel efficient than dual assemblies, as we’ll see when we discuss tire construction. As part of a fuel economy plan, they’re worth considering.