Try walking in deep sand on your favorite beach and you'll have an inkling how under-inflated tires affect fuel economy. It's easy going when you're down near the water and the sand is hard. Go further inland where the Frisbee players and the seagulls hang out and you can feel the difference.
The analogy holds if you flip it upside down - it's not soft, squishy roads we're talking about, but tires. Think of the extra effort required to propel a 200-lb biped through deep sand. Out on the road, the extra energy required to overcome rolling resistance on a 40-ton semi running on soft tires comes straight out of your fuel tank.
Where the Rubber Meets the Road
Rolling resistance derives from three factors: the rubber compound used to make the tire, the amount of flex in the sidewall, and the pattern and thickness of the tread. Rubber compounds have changed a lot over the past 20 years equalizing the balancing act between traction and tire life. Advanced formulations in tread rubber yield longer tread life, better fuel economy, and good traction. But we can go overboard on traction if we're not careful.
True off-road tires, with their deep lugs, won't last long in over-the-road service. But even some lug-style highway drive tires offer more traction than might be needed. According to Goodyear's marketing communication manager, Tim Miller, many Canadian long-haul operators could reduce fuel consumption by running less aggressive, more fuel-efficient rib or rib-lug drive tires.
"An aggressive pattern may look good to a driver, and give him a sense of security knowing he'll be able to pull out of a yard that's covered with snow," Miller says, "But frankly, you don't need that much aggressiveness in the tread pattern to make it work."
A careful evaluation of where you run could change your tire spec'ing decision. Chances are you'd get by with a rib-lug combination pattern, which improves rolling resistance with only a minimal traction sacrifice, Miller suggests. Look at some of the big fleets that run continent wide - like Schneider.
Lug treads tend to be deeper, too, which comes at a cost to fuel economy.
"As tires wear down, the fuel economy gets better and better. Tread accounts for between 60% and 70% of a tire's rolling resistance," says Bridgestone Bandag's Guy Walinga. "A typical new rib tire will have a tread depth of about 18/32 to 22/32. Lug tires offers something like 26/32 to 30/32 of an inch of rubber. We've found a 7/32 reduction in tread depth produces a 10% improvement in rolling resistance."
Bridgestone claims a new rib tire is about 6% more efficient than a new lug tire, and well-worn rib tire (4/32 in.) is about 7% more efficient than a new rib tire.
"There are losses in energy as the tread pattern squirms as it goes through its footprint. Energy is required to work the tire like that, and that energy comes from the fuel tank," Miller says.
Fuel Economy Under Pressure
It's accepted, understood, and agreed that under-inflated tires are a fuel economy liability, but the extent - and the cost - of under-inflation is staggering. In a study published in July 2005 titled, "TECH BRIEF: Commercial Motor Vehicle Tire Pressure Sensor," the U.S. Federal Highway Carrier Safety Administration concluded that only 44% of all commercial vehicle tires were within 5% of their target inflation pressures, with as many as 7% out by 20 psi or more. That suggests a significant amount of fuel is wasted everyday overcoming unnecessary rolling resistance.
"It's hard to tell if a tire is going down with today's steering and suspension systems. The feel of the vehicle doesn't change significantly, even on steer tires," says Dana Spicer's product manager for tire management and wheel-end systems, Jon Intagliata. "Studies have shown that you can take tire pressure down to 20-30 psi on steer tires and drivers could not tell the difference from inside the cab while driving."
The rule of thumb is one percent degradation in fuel economy for every 10 psi under optimum pressure across the vehicle, but as maintenance items go, keeping tires properly inflated is like herding cats. They don't stay where you want 'em for long.
In addition, mis-matched pressure between dual tires was found to be a significant contributor to poor fuel economy and tire maintenance costs. Approximately 20% of all tractor dual tire assemblies have tires that differ in pressure by more than 5 psi, FMCSA notes, while one out of four trailer dual assemblies (25%) have tires that differ in pressure by more than 5 psi.
A recent case study by Bridgestone found that a mere 5-psi difference in pressure between the dual tires creates a differential in the circumference of two tires of as much as 5/16 in. Even that small a difference will cause drag on the larger tire (higher pressure) equal to about 13 ft per mile, or 86 miles of drag for every 35,000 miles of operation.
Nobody relishes the task of maintaining tire pressure, but as the price of fuel - and tire raw materials - continue to skyrocket, the need for proactive tire care has never been more obvious.
Keep up the Pressure on Fuel Economy
It takes 30 minutes to check and maintain tire pressure on an 18-wheeled vehicle - and it's a royal pain in the butt. Today there's hardware that can solve some of our inflation problems.
On the low-tech end of the scale, we have tire pressure equalizers such as Cat's Eye from Link Manufacturing and Crossfire from Dual Dynamics. These provide a visual indication of tire pressure while equalizing the pressure between dual tires. For about $250 for a tandem grouping, they're an inexpensive option, but they still require monitoring and driver intervention if there's a pressure drop.
Several medium-tech solutions exist to monitor tire pressure electronically, such as the Doran 360HD from Doran Mfg., and Pressure Pro from L&S Safety Solutions, or SmarTire, Tire Sentry, SecuTire, and others. Each offers at least a low pressure alert for individual tires. Some have visual pressure displays, all are wireless, and some require additional antennae for trailer service. Prices range from $700 to $1200 and are compatible with most trucks.
At the higher end of the tech spectrum, we find Dana Spicer's SmartWave TPMS (Tire Pressure Monitoring System). SmartWave collects pressure and temperature data from inside the tire, and transmits to a chassis-mounted receiver connected to an external display or through OEM smart dash.
The key, says Intagliata, is to alert the driver before the pressure loss becomes critical. "If the driver is aware of a problem, he can take steps to correct it."
