How much do tires really affect fuel economy?

Tires are not the largest consumer of fuel – we learned in the previous articles that air resistance and weight are the biggest. But tire rolling resistance is just one of the forces holding back a truck. And if rolling resistance holds back a truck, it means the vehicle must generate additional energy to overcome it. This makes tires a fuel consumer.

For this reason, many fleets have found fuel-efficient tires are relatively easy to substitute for regular tires, and don’t require any more effort to maintain.

Before we go further, what is rolling resistance?

Rolling resistance is a way of measuring how much work you’ll have to do – or how much energy you have to use – to roll a loaded tire down a road. You may hear it called “friction” or “rolling drag.”

Since we’re discussing large truck fuel economy, it’s safe to assume a measurement of rolling resistance involves rubber tires and a concrete or asphalt road surface. But consider how different tire materials and road surfaces affect rolling resistance. For instance, pulling the same loaded tire down a sandy trail would take considerable work to compensate for the loose surface.

Incorrect tire pressure, especially underinflation, noticeably affects rolling resistance, even if it’s a hard surface.

It's easy to see this two-wheel cart is difficult to push; it has very poor rolling resistance

How so?

The air inside the casing carries the load.

Here at the Real Answers magazine offices, we pulled a forgotten two-wheel cart from storage and loaded it up with several heavy boxes. Unfortunately, most of the air had leaked from the cart’s tires, so the weight of our boxes squashed them nearly flat.  The cart bogged down and it was practically impossible to push.

For optimum rolling resistance, tires must be properly inflated, whether your vehicle is a two-wheel cart or large truck.

Does an improvement in rolling resistance always
improve fuel economy?

A change in rolling resistance does not produce an equal percentage
change in fuel usage.

For example, if rolling resistance accounted for about one quarter to one third of truck fuel consumption, then an improvement of 5 percent in rolling resistance would only produce about a 1.3- to 1.7-percent improvement
in fuel economy.

[1/4 x 5% = 1.3% and 1/3 x 5% = 1.7%]

You may be surprised to find that tires are responsible for up to one third
of a large truck’s fuel economy. Once, tire rolling resistance accounted for only 15-20 percent of total fuel consumption. As truck designs became more aerodynamic, tires came to represent between 25 and 35 percent of
the fuel used.

If tire designs are improving, how can that be?

Air resistance will always be the biggest consumer of fuel. What’s happening now is truck manufacturers have made great strides in lowering a truck’s drag coefficient, so the contribution for tires is increasing.

So what makes tires more fuel-efficient?

Tires with low rolling resistance use fuel-efficient tread compounds and fuel-efficient casings to promote superior fuel economy.

We’ll examine tread compound and tread designs in the next issue of this magazine. For now, let’s turn our attention to the design of the casing.

Casing design contributes to about 50 to 65 percent of the rolling resistance of the tire.

The biggest factor is energy losses resulting from the deformation of the tire under load. 

The casing plays a big role in helping the tire retain its shape so the tread lies flat against the road for even wear. When the tire flexes as it rolls into and out of contact with the road, the flexing of the sidewall generates heat, which consumes energy.

Anything done to reduce heat generation in the casing as it flexes will reduce rolling resistance and improve fuel economy.

Can a super-low rolling resistance casing be built?

It’s possible, but might not be practical because the tire would probably be extremely rigid. The ride could be tremendously rough. If the drivers weren’t thrown from their seats, the cargo might be battered to smithereens and destroyed.

Second, the truck would probably fall apart, possibly starting with the suspension system.

What about retreadability?

It’s doubtful a rigid casing could live long enough to be retreaded. A rigid casing will very likely suffer impact damage from potholes, curbing, road debris and any other hazards lying in its path.

So what’s the answer?

Continuing to fine-tune the casing components and shape to keep the tire as cool as possible. Lower tire temperatures reduce rolling resistance and improve fuel economy – not to mention increasing casing life and improving retreadability.

Can fleets do anything to reduce rolling resistance?

Keep tires as cool as possible by looking
after air pressure – remember our mushy
two-wheel cart example. The manufacturer
has done everything it can to optimize the
fuel efficiency of the casing – provided you
play your part by putting in the right amount
of air to give it the optimum shape.