REAL Answers Ask the Doctor

The Tire Doctor Responds:

Because it’s the tread that’s responsible for most of a tire’s fuel consumption. So when there’s less tread, there’s less rolling resistance. And that means tire fuel efficiency is not constant over the life of a tire. In fact, as we’ll see, as they approach wearout, most tires are pretty much the same in fuel economy. And in some cases, a tire that claims to be fuel-efficient may actually cost more per mile—even when fuel savings are considered—than a tire that emphasizes long mileage.

How can a tire “consume” fuel?
It doesn’t, really. At least not in the sense that fuel physically goes into the tire. But since it takes energy to overcome rolling resistance, and since that energy has to come from fuel, a tire does, indirectly, consume fuel.

What is the tire doing that consumes energy?
Part of it is just rolling. If you’ve rolled a tire across a service bay, you know that it takes some energy to do that. Imagine what it takes with a load of several thousand pounds—at highway speeds. And that would be true, even if the tire were made of steel or concrete. With a modern pneumatic tire, lots of other things are going on, too.

For example?
Both the tread and the sidewalls are flexing, and individual tread elements may be flexing and squirming. It takes energy to do all that, energy that comes from fuel. After all, the tire tread goes from being circular to flat and back again as it rolls through the footprint. And the sidewalls bulge between the road and axle as the tire turns. Even the tread elements are flexing and squirming as they come into contact with the road. All of that takes energy.

Can’t squirm and deformation and flexing be reduced?
To some extent. A concrete or steel tire wouldn’t flex very much, but it would be a very rough ride. And, you might not be happy with the mileage—or the traction, or the road damage. But squirm, deformation and flexing can’t all be eliminated without compromising other performance factors, like ride, traction and tire life.

How much of a tire’s fuel consumption is the result of the tread?
Most of it, actually. Depending on casing and tread design and compounding, the tread can account for 60-70 percent of the fuel consumption attributable to a tire. So, since the tread is “consuming” most of the fuel energy, as the tread wears away, fuel economy naturally improves.

Then wouldn’t a “fuel-efficient” tire be even better near wearout?
Afraid not. What testing shows is that the fuel efficiency resulting from wear doesn’t simply add to tread fuel efficiency. If some fuel efficiency came from special tread compounding, as the tire wears, that benefit gradually decreases. At the same time, fuel efficiency resulting from wear gradually increases. The two effects seem to offset each other somewhat. Overall, as tires wear, their fuel efficiency gets closer and closer to the fuel efficiency of their casings. And, since most casings are about the same in that respect, the differences between fuel-efficient and other tires tend to disappear.

But we ARE saving while the tire is new, aren’t we?
It depends on how you define “saving.” Bridgestone engineers have been studying this effect for nearly two decades, and just this past year conducted some rigorous fuel economy tests.

The results show that while fuel-efficient tires might save some fuel when new, if you calculate total cost per mile over the entire life of the tire, a tire that lasts longer may actually save you more—even when fuel costs are factored in.

How can that be?
Some fuel-efficient tires tend to wear out faster. To calculate cost per mile, you divide cost by mileage—so if the mileage goes down, cost per mile goes up. And, in the research Bridgestone did, the remarkable result was that for cost per mile, removal mileage was more important than fuel efficiency.
Only 1.5 percent? That’s really not much of a difference.
It certainly isn’t. Nevertheless, in these days of booming freight business and relatively low fuel prices, tires that last longer can mean more productivity and uptime, both of which contribute directly to profitability. As we’ve always said, the real measure of the worth of any piece of equipment, including a tire, is in its return on investment. Every fleet is different, but when you evaluate things for yourself, you’re more likely to make these investments good ones.