Rolling Resistance
Reducing it is a hedge against fluctuating fuel costs.
Mark Putz
Associate Editor
A complex variety of decisions play into improving a truck's overall fuel economy. But spec'ing tires with reduced rolling resistance is an easy way to save cash on fuel in today's uncertain climate.
One way to reduce rolling resistance is reducing tire size. Over the past few years, there has been a trend toward low-profile sizes. According to Continental, rolling resistance is reduced by approximately 5% with low profile sizes, resulting in a 1-2% gain in fuel economy. So if low-profile tires are an option for your application, give them a hard look.
Whether low profile or standard, the good news about spec'ing a tire touted by its manufacturer as a fuel saver is that this tire will also feature a premium casing. Tests by Bridgestone found that casing type factored as roughly one-third of a tire's rolling resistance, and advances in casing design led to improved fuel efficiency. And premium casings lead to better retreadability. In other words, aside from negligible sacrifices in traction and treadwear, there are few downsides to spec'ing fuel-saving tires for most applications.
Reducing rolling resistance has always been a fundamental ingredient in any line-haul fleet's spec'ing recipe. These operations have traditionally avoided aggressive tread designs, which exhibit greater rolling resistance and thus lower fuel economy. But urban operations have also eyed rolling resistance, even if the fuel savings can be more difficult to measure during stop-and-go city driving. That's part of the reason Goodyear says it has engineered reduced rolling resistance into its entire line of tires. Instead of offering a few tires touted as fuel savers, Goodyear says it builds the best possible balance for reduced rolling resistance, given other demands on each type of tire.
Pick up and delivery fleet managers will want to keep in mind that roughly half the touted savings seen in controlled tests will pan out in daily driving. This is not to say that the tests aren't accurate — in fact, testing over the years has revealed a crucial aspect of spec'ing fuel-efficient tires: Trailer tires generate most of the fuel saved, depending on application, load and driving type. Testing done by Bridgestone revealed that changing trailer tires to fuel-efficient types produces a larger effect than changing tractor tires to fuel-efficient types. The tests also found that the actual contribution of individual axle positions to fuel economy varies with vehicle configuration. This goes hand and hand with the fact that setback axles can be hard on steer tires. Setback axles allow tighter turns, but this scrubs steer tires — not a good idea when tread depth is lowered to reduce rolling resistance. And, because trailer rubber is ordinarily cheaper than drives or steers, you'll want to first swap out trailer tires. In other words, switching your trailer tires to fuel-efficient types will result in the most bang for your buck.
The traditional trade-off with rolling resistance involves tread depth. Typically, fuel efficiency is achieved to large degree by reducing the tread depth — which can also reduces mileage to removal. This problem is solved by the new design and research tools available to manufacturers. For example, Continental engineers used scanning tomography to study sections of a tire under actual loaded conditions. With this technology, designers created a cooler-running casing. The benefit is two-fold: Cooler-running tires use less fuel; and less heat on the casing means slower rubber degradation, resulting in longer tire life. These advances are built into the seven fuel-efficient tires rolled out last spring by Continental, carrying the Eco Plus name.
It's worth restating that manufacturers claim fuel-saving tires offer excellent retreadability. And Bandag's retread technology enables tread designs that provide favorable fuel performance. The heart of this technology is its Synergy low-rolling-resistance rubber compound.
Finally, don't waste gains in rolling resistance by neglecting inflation maintenance. Maintaining the correct pressure is as important as the tires used. Tires under-inflated by 15% will cause fuel penalties of around 2.5%, and a 30% under-inflation will increase fuel consumption by 5%.
