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Tires a big part of fuel efficiency: Malloy

Published February 7, 2013

With gasoline prices on the rise again, fuel economy is a top-of-mind concern for many drivers. It’s also a preoccupation for automakers, who are faced with meeting continuously more-stringent fuel consumption targets for at least the next dozen years.

Much attention is being paid to improving engine and transmission efficiency, adding electrification to the powertrains and reducing weight — all of which can play an important role in reducing fuel consumption.

But there’s another area of technology that can have a significant impact as well: Tires.

By various estimates, from five to 30 per cent of the total energy required to keep a typical passenger car moving in the normal driving-speed range can be attributed to the rolling resistance of its tires (depending on the vehicle, tire and driving conditions).

That’s a startling range in the context of automakers spending millions of dollars in the pursuit of fractions of a per cent improvement in fuel economy.

If you consider that fuel consumption is a direct function of that overall energy requirement, the important role the tire plays — and the potential it offers for improvement — becomes readily apparent.

According to Transport Canada, rolling resistance is a measure of the amount of energy used by a tire to deform as it rolls through each revolution.

If you’ve ever watched a tire closely on a car moving at low speed, you’ve undoubtedly seen how much its shape changes as each part of the tire approaches and passes through the area in contact with the ground.

You can actually see the sidewalls compress and bulge out. But there is even more going on within the tire.

According to Michelin, there are bending forces acting on the tire crown, sidewall and bead area, compression forces acting on the tread, and shearing forces acting on the tread and sidewall.

The deformation that results causes internal friction that in turn generates heat within the tire. If you’ve ever put your hand on a tire sidewall after an extended highway drive you can appreciate just how hot it can get.

There are potential fuel-economy gains to be made by reducing tire rolling resistance. And both tire manufacturers and automakers are pushing hard to do exactly that.

In fact, the tires on most new cars today have substantially lower rolling resistance than those of just a decade ago.

In addition, most tire makers now offer specific “low rolling-resistance” tires. They are often fitted as standard equipment on hybrids, electric vehicles, and eco-branded variants of conventional vehicle models.

The U.S. Department of Energy (DOE) reports that switching to low rolling-resistance tires can enable fuel savings of 1.5 to 4.5 per cent.

While that amount may not seem like much, if your vehicle has a consumption rate of 10 L/100 km, it could mean you could go as much as an additional 28 km on a typical 60-litre tank of fuel.

Stated differently, it could save you as much as $3.37 every time you fill that 60-litre tank, at today’s gasoline prices.

So why aren’t all vehicles equipped with low rolling-resistance tires? As with all aspects of automotive engineering, there’s always a trade-off.

Typically, the parameters that reduce rolling resistance may also have a diminishing effect on other characteristics such as overall grip, wet-weather performance, or ride quality. It’s all a balancing act to satisfy overall customer needs without compromising too much in any direction.

You don’t have to buy such special-purpose tires to take advantage of the benefits of lower rolling resistance, however. Just keep your tires properly inflated.

Not only can under-inflated tires be dangerous, they can have a dramatic effect on rolling resistance.

To illustrate that point, just try pushing a loaded wheelbarrow with a partially-inflated tire; then try again with the tire fully inflated. You’ll quickly see the difference.

The DOE estimates that a tire with a recommended inflation pressure of 35 psi will have a 12.5 per cent greater rolling resistance if inflated to just 28 psi — a difference probably imperceptible to most drivers.

Which is a very good reason to check your tire pressures regularly and keep them inflated to the pressure recommended on your vehicle’s information label.

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