Tread Effects Casing Effects Inflation Effects

Tires are just one of many factors influencing truck fuel consumption. Fuel-efficient tires are relatively easy to spec or retrofit to improve fuel economy. Only about 1/3 to 1/4 of any improvement to a tire’s rolling resistance actually shows up in scientific fuel economy tests.

Tire rolling resistance is just one of several factors in large truck fuel consumption.

How much do tires contribute to fuel economy?

Since rolling resistance is only a part of the fuel consumption picture, a percentage change in rolling resistance does not result
in an equal percentage change in fuel usage.

For example, if tire rolling resistance accounted for about one-third of truck fuel consumption, then an improvement of 3 percent in rolling resistance would only produce about a 1 percent improvement (1/3 x 3% = 1%) in fuel economy.

What makes this even more complicated is that the share of
fuel consumption attributable to the rolling resistance of tires
can change.

In the early 1980s, tire rolling resistance accounted for about
15–20 percent of total fuel consumption. But by the 1990s,
it represented 25–35 percent of fuel used.

You’d almost think—from those figures—that tires were getting worse. What really happened is that engines and aerodynamics got so much better that tires became a bigger factor.

Interestingly, faster highway speeds may be reversing this trend. At speeds of 70–75 mph, tire rolling resistance probably only accounts for 15–20 per-cent of fuel consumption.

Tire design and construction effects

Radial tires are one of the most significant improvements in tire technology—and in tire fuel economy. In early tests, Bridgestone found that converting from bias-ply tires to radials improved fuel economy more than 10 percent. This represents about a 30–40 percent decrease in rolling resistance.

Radial tires have about 30–40 percent less rolling resistance than comparable bias-ply tires, and can therefore improve fuel efficiency by about 10 percent. Most of the rolling resistance of a tire is the result of tread design and compounding. Cap-Bas tread construction can help reduce rolling resistance without compromising casing durability.

And, improvements in radial tire design and construction continue. Just as with the different components of a truck, the different parts of a radial tire make different contributions to the rolling resistance and fuel efficiency of that tire.

There is friction between the tire and the road (largely a result of tread design and tread compound), tire air resistance, and energy losses resulting from the deformation of the tire under load and internal stresses within the tire.
About two-thirds of tire rolling resistance is the result of tread design and tread compounding.

About two-thirds of tire rolling resistance is the result of tread design and tread compounding.

Tread compound effects

Most of the rolling resistance of a tire, about 60–70 percent, comes from the tire tread. For that reason, most manufacturers’ efforts have focused on tread compounding.

Some compounds, especially those incorporating silica or using special formulas that combine natural and engineered-structure synthetic rubber, can reduce tire rolling resistance significantly.

In some cases, a two-layer, or “cap-base” tread can be used. The cap compound, which is nearest the road, is chosen for high resistance to abrasion, for long tread life and high traction on wet roads.

Since compounds with these characteristics tend to generate more heat, a base layer between the cap and the casing is chosen with a cooler-
running compound—to protect the casing from heat buildup.

The result is lower overall tire temperature for longer casing life and better traction, tread life and retreadability. Lower temperature also means lower rolling resistance and better fuel economy, because less fuel energy is wasted as heat.

By combining a tough, fuel-efficient cap layer with a cool-running base layer, some fuel-efficient radials can provide excellent fuel economy without compromising traction, tread life or retreadability.

Tread depth effects

Tread depth has a significant effect on tire fuel economy. Bridgestone tests show that as a tread wears, the fuel efficiency of the tire usually increases. The increase is fairly rapid at first, then slows as the tire approaches wearout.

The shallower the tread, whether by design or as a result of wear, the better the fuel economy. As they approach wearout, most tires tend to have very similar fuel efficiency, even when regular and fuel-efficient types are compared. Shallow rib designs tend to be the most fuel-efficient.

So, one of the easiest ways to make a fuel-efficient tire is to cut down on tread depth. Another way of looking at it is to say that during the last half of its useful tread life, just about every tire is fuel-efficient.

As tread wear progresses, the fuel effiency of most tires improves.

Tread wear effects with fuel-efficient tires

Why do shallow-tread and worn tires save fuel? Partly because with less tread, they weigh less, and because the shallower tread is less subject to energy-wasting squirm.

But what’s interesting is that some fuel-efficient tires actually provide very similar fuel economy to non-fuel-efficient types
as they approach wearout.

Since most of the fuel economy of a tire is in its tread, as a fuel-efficient tread compound is worn away, the fuel economy of the tire becomes more and more dependent on the fuel efficiency of the casing.

Basically, what happens is that the final rolling resistance is more or less a measure of the rolling resistance of the casing. And some casings have a lower rolling resistance than others.

Retreads made with these casings will have inherently lower rolling resistance throughout their lives than retreads made using less fuel-efficient casings.

Tread design effects

Tread design affects fuel economy too. As a general rule, shallow treads are more fuel-efficient than deep ones. And, rib designs tend to be more fuel-efficient than lug or block designs.

This suggests that if you can achieve sufficient traction, the most fuel-efficient radials might be shallow-tread rib designs. Bridgestone and other manufacturers make rib radials that are suitable for use in all wheel positions, and which can produce significant fuel economy advantages.

Once again, however, you will need to balance your needs for long original tread life and pulling power before switching to rib-type radials.

As they approach wearout, differences in the tire fuel effiency tend to disappear.

Fuel economy with retreads

It’s important to remember that when fuel economy is achieved by modifying tread compounds, tread depth or tread pattern, the fuel economy benefits usually end when the casing is retreaded—except where the retreader applies a new, fuel-efficient tread.

Retread fuel economy depends mostly on the retread compound and design chosen. Most retreads are similar in their fuel efficiency. Bridgestone offers radials with fuel-efficient casings that retain part of their fuel efficiency even when retreaded. Changing tire design to save fuel may compromise other critical performance factors. Proper inflation pressure optimizes tire fuel economy, tread life and casing durability—with any tire.

Generally, retreads tend to be very similar in their fuel efficiency. Consult your retread supplier for more information about fuel-efficient tread compounds and patterns.

Casing effects

Casings, as we’ve said, contribute about one-third of tire rolling resistance, but modifying casings to improve fuel efficiency is difficult. Since the casing is the basis for retreading, changing casing design or components could compromise retreadability.

Nevertheless, Bridgestone has used super-computer simulations and finite element analysis to create new casing designs and shapes that minimize rolling resistance without reducing casing durability. By optimizing the distribution of stresses in the casing, fuel economy is maximized.

The result is new casings that retain part of their fuel efficiency even when retreaded, because part of the fuel economy is a function of the casing, not just of the tread. Changing tire design to reduce rolling resistance must be done with care, to avoid compromising other performance factors.

Changing tire design to reduce rolling resistance must be done with care, to avoid compromising other performance factors. 1. Casing Air Volume Increase: Can reduce fuel consumption, but may increase minimum dual spacing requirements. 2. Bead Filler Decrease: Can decrease weight, but may weaken sidewall near bead, decreasing casing durability. 3. Sidewall Gauge Reduction: May save weight, but can increase sidewall flex, diminishing handling and retreadablity. 4. Tread Depth Decrease: May save fuel, but can decrease original tread life, resulting in earlier removal. 5. Tread Compound Change: Can improve fuel efficiency, but may produce faster wear, accelerate irregular wear or sacrifice wet performance. 6. Crown Radius (Tread Flatness) Increase: Flattens tread against roadway, producing a large contact area, which can decrease wet performance.

Inflation effects

We’ve seen that casing shape and stress distribution are critical to both fuel economy and retreadability. That’s one reason correct inflation pressure is so important with any tire, but especially with fuel-efficient radials.

It is not the tire, but the air inside it that supports the load. And, it is the air inside the casing that keeps that casing the right shape.

So, proper inflation pressure for a given tire size and load is critical to maintaining proper stress distribution in the tire. This in turn reduces flexing and heat build-up that can waste fuel and shorten casing life.

It is the flexing of tire sidewalls that generates much of this heat. Proper inflation optimizes the amount of flexing, balancing heat generation with the ability to absorb road shocks. Even with special rubber compounds that minimize heat build-up in sidewalls, correct inflation is vital.

These factors explain one of the reasons wide base radials are more fuel-efficient. Since a single wide base tire replaces a dual assembly, there are half as many sidewalls flexing and generating heat during each tire revolution. Less heat means less fuel consumed.

Regardless of the type of tires you use, maintaining correct inflation pressure for the load will optimize tire performance and fuel economy.

Bridgestone engineers have tested the effect of inflation pressure over a 40 PSI range, from 20 PSI below to 20 PSI above standard. They found a two percent variation in fuel consumption over that range.

Inflation pressure has a definite effect on fuel economy and is something you can begin monitoring and maintaining immediately—regardless of the type of tires you use.

In addition, proper inflation tends to minimize irregular wear for longer tire life.

Overinflation is neither effective nor recommended as a fuel economy method. Tires on different axles make different contributions to tire fuel economy. Axle weight distribution does not accurately predict position contributions to fuel economy. Trailer tires make a bigger contribution to fuel economy than either their number or loads would lead you to believe.

Bridgestone also tested the effects of overinflation. While it did not prove to be an effective way to save fuel, in general, Bridgestone engineers suggest you use the maximum allowable inflation pressure in over-the-highway steer tires for best overall service, including handling and resistance to heat and irregular wear.

Higher drive tire inflation pressures (that do not exceed the maximum allowable) can also result in better overall tread wear.