fuel & tires

Tire Contributions
to the Fuel Bill

So far, we've talked about the two most important factors in large truck fuel economy, speed and load. We've also seen how speed affects fuel-efficient tires, and how tire selection can convert non-paying load into payload.

What about the tire itself? Some tires are more fuel-efficient than others. Tires aren't the largest factor in fuel economy, but they are one of the easiest things to change. That's why a great deal of attention has been focused on using tires to improve fuel economy.

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, 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 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 retreadability.

As both standard and fuel-efficient
radial tires approach wear-out,
their rolling resistance decreases
– and tends to equalize.

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

Wear effect with fuel-efficient vs. conventional radials

Basically, what happens is that the final rolling resistance is more or less a measure of the rolling resistance of the casing.

Some casings have a lower rolling resistance than others, and retreads made with these casings will have lower rolling resistance – and better fuel efficiency – throughout their lives, than retreads made using less fuel efficient casings.

Tread design effects

Tread design changes fuel economy too. Usually, 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 more fuel-efficient tires might be shallow-tread rib designs.

Bridgestone and other manufacturers make rib tires that are suitable for use in all wheel positions, and which can produce significant fuel economy advantages.

There are also exceptions to the rib vs. lug rule. With newer-generation drive tires, incorporating continuous shoulder ribs, tire designers have found they can be a lot more flexible in selecting tread rubber compounds.

The result is you may find certain closed-shoulder drives that offer overall fuel economy equal to or even better than some rib designs.

Once again, computer analysis, like that provided by the Bridgestone Tire Life Cycle Cost program, can help you decide whether you should spec rib tires, lug tires or a mix of both for best fuel efficiency.

Fuel economy with retreads

It's important to remember that when fuel economy is achieved only by modifying tread compounds, tread depth or tread pattern, the fuel economy benefits sometimes end when the casing is retreaded – unless the retreader applies a new, fuel-efficient tread.

There are fuel-efficient retreads available. In fact, Bridgestone Bandag Tire Solutions offers retreads with fuel economy, treadwear and traction competitive with some of the very best new tires, but at a fraction of the cost of new tires.

Consult your retread supplier for more information about fuel-efficient tread compounds and patterns.

Casing effects

Casings (including belts) contribute about 50 to 65 percent of tire rolling resistance.

Bridgestone uses supercomputer 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 fuel efficiency, even when retreaded, because part of the fuel economy is a function of the casing, not just of the tread.

Inflation pressure 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 ones.

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 tires can be 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.

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. And that reduces tire cost per mile too.

Bridgestone even tested the effects of overinflation. While it did not prove to be an effective way to save fuel, in general, Bridgestone engineers say you may use the maximum allowable inflation pressure (displayed on the tire sidewall) in over-the-highway steer tires, for best overall service, including handling, durability and resistance to heat and irregular wear. Drive and trailer tires do not necessarily respond the same way.