You're probably aware that things like vehicle weight, aerodynamics and engine and driveline efficiency are major factors in determining fuel consumption.
But did you know that your tires also account for a significant amount of the fuel your vehicle uses?
Depending on the vehicle, tires and driving conditions, according to various sources, from 5-to-30-percent of the total energy used to keep a typical passenger car moving, within the normal driving-speed range, is expended by the tires.
That energy, which has to come from the fuel, is used to overcome what's known as tire rolling resistance.
Tire rolling resistance
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.
You've probably seen that deformation for yourself if you've ever looked closely at a tire on a slowly-moving vehicle. You can see how much it changes shape as each part of the tire approaches and passes through the area in contact with the ground – which is called the contact patch.
The sidewalls typically compress and bulge out, but there is even more going on within the tire that you can't see.
Specifically, 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.
All those forces cause internal friction that generates heat within the tire, as you probably know if you have ever touched a tire sidewall after an extended highway drive. There's a good reason race-car pit crews wear insulated gloves when they're changing tires!
Given that heat is a form of energy and that energy can be neither created nor destroyed (high-school physics; the Law of Conservation of Energy), in this case some of the energy from the gasoline, diesel fuel or electricity used to drive the vehicle is being transformed into heat in the tires.
Can anything be done to conserve some of that energy that's being wasted as heat?
Keep your tires properly inflated
One thing you can do yourself, right away. Keep your tires properly inflated.
If you've ever tried to push a loaded wheelbarrow with a partially-inflated tire, then tried again with it fully inflated, you can appreciate how much more energy it takes to keep a soft tire rolling.
The U.S. Department of Energy (DOE) estimates that a tire with a recommended inflation pressure of 35 psi will have 12.5%-greater rolling resistance if under-inflated to just 28 psi – a difference probably imperceptible to most drivers, either visually or in terms of driving feel.
All of 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.
Beyond just recommending proper inflation, automakers tire manufacturers are doing their parts by equipping today's new cars with tires that have substantially lower rolling resistance than those of just a decade ago.
"Low rolling-resistance" tires
Going even further, most tire makers now offer specific "low rolling-resistance" tires, which are often fitted as standard equipment on hybrids, electric vehicles, and other eco-focused models.
According to the DOE, switching from conventional to low rolling-resistance tires can result in fuel savings of 1.5-to-4.5%.
That amount may not seem like much, but it's up to one litre out of every 22.5 litres you buy.
Stated another way, it means you could go as much as 30 km further on a tank of fuel in a typical mid-size car. Or save as much as $3-to-$4 on a typical fill-up, driving the same distance, at today's gasoline prices.
Which raises the question, why aren't all vehicles equipped with low rolling-resistance tires? Because, as with all aspects of automotive engineering, there's always a trade-off.
Often, the changes made to a tire to reduce rolling resistance will have a diminishing effect on other characteristics such as overall grip, wet-weather performance, or ride quality.
It becomes a balancing act for manufacturers of both vehicles and tires to satisfy a broad range of customer needs without compromising too much in any direction.
But huge strides have been made in maintaining other performance levels at acceptable while still reducing rolling resistance.
So if fuel economy is your top priority, next time you're shopping for tires, look for those identified as "low rolling resistance."