As record-high fuel prices continue to impact the bottom lines of all types of trucking operations, fleet managers are taking a much closer look at all of the factors that contribute to fuel efficiency. One significant part of the mpg equation is tires, and, as a sizable aspect of any fleet’s costs, maximizing the value of tires is a matter of understanding and weighing a number of factors.
Information supplied by the Goodyear Tire and Rubber Co. outlines some of the factors and trade-offs associated with tires and their affect on fuel economy:
Tread depth Deeper tread extends miles to removal but also increases rolling resistance, which burns more fuel.
Tire profile Low-profile tire sizes are lighter than standard aspect ratio sizes and can enhance fuel efficiency, but rear axle ratio and transmission gearing should be changed if tire rpm increases by more than 3 percent.
Tread type Rib-type tires are the most fuel efficient but don’t have the traction of a lug tread. Fleets can generally specify rib designs on drive tires if they are running a lot of highway miles in warmer climates, but if trucks regularly encounter slippery surfaces, a tread design specifically developed for drive axle applications is required.
Chip and chunk resistance In applications with a lot of off-road operation, tread compounds are needed to help resist chipping and chunking and extend tire life. However, these compounds may not be the most fuel efficient.
Retreads When retreading, fleets can choose to spec a more fuel-efficient tread.
In its “Guide to Large Truck Fuel Economy for a New Millennium” Bridgestone/Firestone North American Tire (BFNAT) addresses the subject of fuel economy and tires, beginning with some questions fleets often ask. For example, the guide notes, how do tires fit into the overall fuel economy picture? In addition, how can fleets select the best tire for their operations?
The answers can be complicated, BFNAT says, because fuel efficiency can involve performance tradeoffs. Making a tire fuel efficient, for example, sometimes compromises wet traction, shortens tread life, increases susceptibility to irregular wear or reduces casing durability.
One of the most often-asked questions by fleets, the manufacturer adds, refers to why results reported in fuel economy tests are so difficult to duplicate in the real world.
“The reason is simply that there are many factors that influence the fuel consumption of a large truck, and most of them interact with each other,” the guide says. “When you run tests, you carefully control as many variables as possible. In the real world, you don’t have the luxury of controlling everything. One day, you may be hauling a full load of steel, and the next, a partial load of potato chips. Weather, roads and terrain also change constantly.”
Another factor, the guide also points out, is that tires change over time. “As tires wear out,” it states, “their fuel efficiency often improves. In fact, the difference between a fuel-efficient tire and a regular tire may completely vanish as the tread wears. That means if your tests were done using new tires, the results may be very different as the tires wear, especially with today’s long-lasting truck tires.”
“If fleets are really concerned about their bottom lines, they should be looking at all these factors and putting the focus where it needs to be,” says Tim Miller, Goodyear’s marketing communications manager. “Fleets intent on getting the best fuel mileage possible need to find the right blend of new tires and retreads to maximize the bottom line for their specific operations. As fuel costs skyrocket, changes may need to be made to rebalance tire-related operating costs.”
Regardless of make or model, tire size, style or manufacturer, one fact is indisputable proper tire inflation pressure is the most important thing a fleet can do to improve fuel efficiency. An underinflated tire increases rolling resistance because there’s more deflection or flexing of the sidewall.
That consumes more fuel by requiring the engine to work harder. By some calculations, every 10-psi drop in inflation pressure results in a one percent drop in fuel economy.
Fleets are increasingly putting the focus on maintaining proper inflation pressures; one option available to them is automatic tire inflation and monitoring systems. This technology was recently validated in a research project conducted by the U.S. Department of Transportation’s Federal Motor Carrier Safety Administration (FMCSA) on the design, functionality and performance of tire pressure monitoring and inflation systems for commercial motor vehicles.
“The act of tire pressure maintenance is labor and time intensive,” the FMCSA report notes. “An 18- wheeled vehicle can take from 20 to 30 minutes to check all of the tires and inflate perhaps two or three tires that may be low on air. To complete this task once each week on every tractor and trailer becomes a challenge for many fleet operators. As a result, tires are often improperly inflated.”
Conclusions by FMCSA about fleet tire maintenance practices include the following:
Approximately 7.08 percent of all tires are underinflated by 20 psi or more. Only 44.15 percent of all tires are within 5 psi of the target pressure.
For-hire carriers (truckload and less than truckload), and owner-operators generally have better tire inflation maintenance practices than private carriers. As a group, for-hire carriers had 7.01 percent of all tractor tires underinflated by 20 psi or more. In contrast, private carriers sampled had 13.21 percent of all tractor tires underinflated by 20 psi or more.
Tire inflation maintenance practices correlate closely with the size of the fleet. For tractors, fleets with 50 power units or fewer have 19.07 percent of their tires underinflated by 20 psi or more, while fleets of greater than 3,000 power units have only 2.06 percent of their tires underinflated by 20 psi or more.
Tractors and trailers have a significant challenge with mismatched dual tires. Approximately 20 percent of all tractor dual tire assemblies have tires that differ in pressure by more than 5 psi. One out of four trailer dual assemblies (25 percent) have tires that differ in pressure by more than 5 psi.
With reduced fuel economy cited as a consequence of incorrect inflation pressures, one of the objectives of the FMCSA research was to evaluate the potential benefits of tire pressure monitoring sensors and automatic inflation systems. Key observations from the cost-benefit analysis by FMCSA include that, for typical TL and LTL fleets, fuel economy loss due to improper tire inflation is about 0.6 percent. “That translates into an increase in annual costs for both new and retreaded tires of 10 percent to 13 percent,” the agency states.
FMCSA goes on to quantify its return on investment analysis. “If tire pressure monitoring and automatic inflation systems available for commercial vehicles could be installed for approximately $1,000 per tractor-trailer combination, return-on-investment periods for an average fleet would be between one and two years,” the report says. “Even for fleets with relatively good tire maintenance practices, fleets which would demonstrate a 25 percent reduction in total cost of improper inflation compared to average, the cost-effectiveness of tire monitoring and automatic inflation systems is still within return periods of less than three years.”
The FMCSA analysis strongly suggests that “the savings potential from tire pressure monitoring and automatic inflation systems could support the purchase price.” Today, suppliers of commercial vehicle automatic tire inflation pressure and monitoring systems are making several technologies available to fleets.
The Meritor tire inflation system (MTIS) by PSI monitors and maintains air pressure at constant and proper levels using the trailer air supply routed to a control box and then into each axle. Acting as a conduit, the axles carry air through a rotary union assembly at the spindle end, which then distributes the air to each tire as needed. A pressure protection valve ensures the integrity of the brake system, and if a tire failure occurs, check valves in the delivery lines prevent loss of pressure from the remaining tires. The system, which can be installed on any type of trailer, also features an indicator light that informs the driver of excessive air-pressure loss.
Dana Corp.’s Commercial Vehicle Systems group recently launched the SmartWave TPMS (tire pressure monitoring system). SmartWave TPMS provides real-time tire pressure monitoring by sending pressure and temperature data to off-board communication systems. The system also features a recordkeeping function and has a temperature-monitoring feature, so it can automatically compensate for fluctuations and ensure proper inflation regardless of tire temperature. SmartWave TPMS also alerts the driver via a warning lamp if a tire deviates from the manufacturer’s recommended pressure. Each tire’s condition also can be graphically indicated on a dash display unit, allowing the driver to see the current pressure of each tire.
Dana’s portfolio of tire pressure management solutions also includes the Spicer TIMS tire inflation and monitor system that is available as a standalone system or integrated with Bendix TABS-6 trailer ABS. In addition, the manufacturer offers the Spicer TPCS tire pressure control system with inflate and deflate capability to improve mobility on soft soil, severe grades, sand and snow. TPCS, available as a factory-installed option, also features an integrated driver display module and is operated with dash-mounted rocker switches.
With the eTire system from Michelin North America, tires can be tracked electronically using hand-held and drive-by readers, which are designed to provide accurate tire pressure maintenance information instantly. The pressure information is accurate, the manufacturer says, because the readers account for cold equivalent pressure, giving precise pressure readings under any conditions.