After all the money you pay for gasoline, you’d probably be surprised to learn how little of it actually powers your vehicle. It’s estimated that only about 15 per cent of the fuel’s energy moves the car down the road, with the rest lost to such things as overcoming aerodynamic drag, wasted heat, idling losses, accessories, and of interest to us here, the tires.
A little more than 4 per cent of your fuel’s energy is used just to overcome the rolling resistance of the tires. Add that up over all the petrol you pump into your vehicle over its lifetime, and you could probably think of much better ways to spend that money.
While rolling resistance has always been a factor in tire design, it has recently moved up in importance. That’s partly because of consumer demand for low-rolling-resistance (LRR) tires, but primarily, it’s because automakers have to meet ever-tightening government standards for fuel economy, and they’re specifying improved rolling resistance in the tires they commission as original equipment on their vehicles.
The problem manufacturers face is that whenever a tire’s attributes move in one direction, there will be compromises in other directions. When hybrids first hit the market, they were shod with LRR tires that were more fuel-efficient than regular tires, but they were noisy, they didn’t have much traction on wet roads, they wore out quickly, and the ride was harsh.
Today, there are far fewer compromises with LRR tires, thanks to such things as new tread compounds, tire construction, and optimized tread patterns. “Comfort and noise levels have been improved in low-rolling-resistance tires,” says Ron Margadonna, senior technical marketing manager for Michelin North America. “We engineer the fuel saving, but there is no compromise in safety for wet or dry braking, or longevity. The key is managing that energy as the tire deforms through its revolution, so there are advanced polymers that we’re using.”
For most manufacturers, LRR has become a technology added to almost all tires, rather than a fuel-efficient tire line itself, although some do make ultra-LRR tire lines for a specific segment of the market. Michelin, for example, uses LRR in most of its tires, marking a “Green X” on the sidewall of those that incorporate the technology. But it also markets an ultra-LRR line, called the Energy Saver, aimed primarily at hard-core mileage fans such as those who drive hybrids or electric vehicles. Because the tire is weighted so heavily toward LRR, there are compromises: its wet traction is lower and it doesn’t last as long.
Even at the other end of the scale, many performance tires are being designed for improved LRR, since even at the premium level, drivers don’t want to be constantly visiting the gas station. Pirelli, for example, incorporates LRR technology into most of its tires, save for the ultra-high-performance lines, where traction becomes the primary goal.
Within the next couple of years, the U.S. National Highway Traffic Safety Administration (NHTSA) will introduce easy-to-understand tire labels for consumers, which you can expect to see here. The labels will outline a tire’s fuel efficiency and emissions rating, its wet traction, and its tread wear. NHTSA currently mandates the Uniform Tire Quality Grading, or UTQG, which includes tread wear, traction grades, and temperature resistance, but deciphering them requires specialized knowledge. These grades, molded into the sidewall, are rated in numbers for tread wear, and letters for traction and temperature. The design for the new labels is still under assessment, but the most likely final choice will be either a five-star rating system, or coloured bands. The labels will be similar to the new European Union tire labels that will be required overseas later this year, but instead of the tread wear rating, the EU labels will measure the tire’s decibel level when in use, to meet EU standards for road noise.
Of course, buying an LRR tire is only half the equation. The other half is in maintaining it so it’s able to return the attributes the company promised. Rolling resistance increases when tire pressure is low, so once a month, check your tires. The recommended pressure is found on a label in your vehicle’s door jamb, or sometimes inside the glove box or fuel filler door, or in the owner’s manual. (Don’t go by the pressure marked on the tire, which is the maximum it can hold.) Some people overfill their tires, since this can further reduce rolling resistance, but it’s not recommended, since the tires will wear unevenly and the ride will be excessively harsh.
It’s also a good idea to have your vehicle’s alignment checked at least every two years. If a tire’s caster or camber is off, the resulting drag will affect your fuel mileage. And finally, replace your tires any time the tread is worn (you can use the penny test), or there are bulges or cracks in the sidewall. Replacing worn tires not only helps with fuel efficiency, but keeps you safe as well.