If you are like nearly one-third of American motorists, the last time you checked the air pressure in your tires was too long ago, and at least one tire is dangerously under-inflated.
Under-inflated tires run hotter, wear faster and have greater chance of suffering tread separation and blowouts. The National Highway Traffic Safety Administration cited under-inflation as a cause of tread separation on Firestone tires, which has resulted in 271 deaths, many in Ford Explorers.
That is one reason Congress two years ago, reacting to the Firestone recall and Explorer rollovers, mandated auto safety measures including tire pressure monitors on new vehicles.
Under rules NHTSA issued in May, new vehicles must have tire pressure monitors (TPMs) under a phase-in schedule beginning Nov. 1, 2003. NHTSA will let automakers use two types of monitors the first three years–direct and indirect.
Direct systems have individual pressure monitors on each wheel that transmit a radio signal to activate a warning light when any tire differs significantly from a recommended inflation level. Direct TPMs provide pressure readings whether a vehicle is moving or parked. NHTSA’s rule requires direct systems to give warnings when any tire drops 25 percent below normal. For a tire that requires 32 psi, that would be 24 psi.
Indirect systems operate off a vehicle’s anti-lock brake system and measure differences in rotational speeds among the tires. They work only when a vehicle is moving. NHTSA’s rule requires indirect TPMs to warn when one tire is 30 percent or more below the others. So for that same 32-psi tire, that would be 22 psi.
The Tribune tested two versions of each system that are available, and the direct monitors proved more effective and informative than the indirect type.
With the direct TPM General Motors offers on the Chevrolet Corvette, which uses run-flat tires, and Cadillac Seville and DeVille, the test car, the tire-pressure warning came on after driving less than three blocks if any tire was at least seven pounds below or above the recommended inflation pressure.
A dashboard display says to “Check Tire Pressure,” and the driver has to scroll through a series of messages to find the pressure reading for the affected tire. The driver can turn off the display, but it comes on again every time the engine starts until the pressure is corrected. It automatically goes off once the air pressure is back to normal.
A direct TPM the Chrysler Group offers on the Chrysler 300M, which was tested, and Town & Country, Dodge Caravan and Jeep Grand Cherokee activated a warning after less than a block of driving.
A display on the overhead console shows pressure readings for the four tires, and any that are too low or high flash. The warning came on when pressure was more than six pounds below or more than 14 pounds above normal.
The warning goes off after 60 seconds but comes on every time the engine starts, and it goes off automatically once inflation is back to normal.
The GM and Chrysler direct systems warn when more than one tire is under- or over-inflated and provide constant pressure readings. Among other manufacturers that offer similar direct TPMs are Ford, Mercedes-Benz and Nissan.
GM also offers the same indirect TPMs on several Chevrolet, Buick, Pontiac and Oldsmobile models, and that system was less effective.
The indirect TPM on a Chevrolet Monte Carlo activated a prominent warning display on the dash after about four blocks of driving when one tire was 12 pounds below normal inflation. Like other indirect TPMs, GM’s system does not specify what tire is low, so the owner has to check all four.
GM’s owner’s manual cautions that the indirect TPM “won’t alert you if the pressure in more than one tire is low, if the system is not properly calibrated or if the vehicle is moving faster than 70 m.p.h.”
BMW offers a direct TPM on its 7-Series sedan and an indirect system on several models. The indirect TPM on a BMW M3 didn’t activate a small, red dashboard warning light until a tire was driven more than two miles at 50 percent below normal inflation.
BMW spokesman Gordon Kiel said the system is designed to activate when a tire drops 30 percent below normal inflation but couldn’t explain why it wouldn’t work until one tire was 50 percent low.
The M3 owner’s manual calls the system a “flat-tire monitor” that “detects whenever the inflation pressure in a tire drops off significantly.”
The BMW and GM indirect systems have to be reset after inflation is brought back to normal and after the tires are rotated. A dashboard button resets BMW’s. GM’s is more complicated.
The Monte Carlo’s owner’s manual says one way to reset the system is to turn the headlight switch from off to the parking light position three times, which did not work in two tries. Another way is an eight-step procedure using various radio controls, which took longer but turned off the light.
The direct TPMs from Chrysler and GM need to be reset after the tires are rotated, and they require a specific tool that dealerships and tire dealers should have.
Ford and Toyota are among other manufacturers that offer indirect TPMs on certain models. All indirect systems have similar limitations because of the technology.
NHTSA’s staff originally advocated the direct system but compromised under pressure from the White House and the auto industry to allow automakers to use both for three years. But the rules are being challenged on two fronts.
Safety advocates have sued NHTSA to require direct monitors, and the tire industry charges that NHTSA’s rules will increase the number of vehicles running on unsafe tires.
Public Citizen, the Center for Auto Safety and the New York Public Interest Research Group have sued NHTSA, charging the indirect monitors will do little to warn consumers and prevent accidents. The suit seeks to force automakers to use the more expensive and sophisticated direct system.
NHTSA estimates that adding the indirect type to a vehicle with ABS will cost automakers about $13 per vehicle and the direct system will cost $70.
The auto industry does not disclose its costs but suggests that NHTSA’s estimates do not include engineering and development costs.
Joan Claybrook, president of Public Citizen and a frequent auto industry critic, calls the indirect type useless because it does not warn if all four tires lose air pressure at a similar rate, even to the point of almost being flat. Nor does it provide a constant pressure reading, which gives the driver an early warning that a tire is getting low.
Claybrook said allowing automakers to use the cheaper indirect system initially means it will become the standard in three years.
“Once the manufacturers decide they’re going to use the indirect system, they won’t want to change and retool for something else,” she said. “They’ll fight it even harder than they are now. The final rule will be based on how many manufacturers use which system.”
The Rubber Manufacturers Association, which represents the tire industry, has petitioned NHTSA to revise the rule, contending consumers will not be warned early enough that their tires are low. Tiremakers say under-inflated tires cannot carry as much weight as fully inflated tires and are more apt to fail under a heavy load.
Dan Zielinski, a spokesman for the Rubber Manufacturers Association, said if TPMs don’t warn drivers until tires are 25 to 30 percent below normal, “they will go off after the tires have reached an unsafe condition.”
A tire with a recommended pressure of 32 can drop to 26 without triggering a warning, he said, but it could “be insufficient to carry a vehicle’s maximum load,” what it can carry in passengers and cargo.
For example, P225/60R16 tires can carry 1,521 pounds with the air pressure at 32 but only 1,378 pounds at 26. A minivan or sport-utility vehicle with seven occupants and cargo may exceed 1,378 pounds.
“What concerns us is that the government is essentially validating that it is OK to drive on tires that are dangerously under-inflated,” Zielinski said. “By definition of NHTSA’s rule, you’re not in danger as long as that warning light is off. There may be accumulative damage that causes sudden tire failure.”
Tiremakers want NHTSA to change the rules because they fear being sued over deaths and injuries caused by under-inflated tires.
“If someone’s tire fails without them seeing a warning light, an enterprising trial lawyer will see a defect in the tire,” Zielinski said.
The tire industry wants NHTSA to mandate inflation pressures that have enough reserve so a tire can carry its maximum load even if it is under-inflated.
“When a low-fuel warning light goes on, you still have gas in your car. When the tire pressure warning light goes on you should have enough air pressure to carry a full load,” he said.
Automakers can raise the recommended tire inflation or use larger tires that can carry heavier loads, he added. Direct TPMs also can be set to warn drivers sooner, such as when a tire loses 10 or 15 percent of its air pressure.
NHTSA spokesman Rae Tyson said the agency agreed to allow both systems because it had to develop a TPM regulation under a congressional deadline.
“The two systems give the auto industry a choice, and this represents a good compromise. It will provide the consumer a degree of protection,” he said, declining further comment on the challenges to NHTSA’s rules.
The auto industry says the indirect system will put TPMs on more vehicles before the federal mandate because it piggybacks on ABS, while the direct type requires re-engineering vehicles to accommodate the technology. NHTSA estimates that 67 percent of new vehicles have ABS.
Vann Wilber, safety director for the Alliance of Automobile Manufacturers, a trade group that represents most vehicle manufacturers, disputes Claybrook’s assertion that TPMs should be surrogate tire pressure gauges.
“We have an issue with that. The system should tell owners when they are getting an unusual tire leak. It is not intended that you should never, ever check your tire pressure,” he said. “Our message is don’t just ignore your tires because you now have a warning light.”
The alliance estimates more than 2 million vehicles on the road have tire monitors, the bulk of them GM models with the indirect type.
