Have you ever seen a vehicle with one or more tires that appear noticeably
low on tire pressure? Didn't you want to warn the driver of the situation
before that slight inconvenience became a calamity? What if the vehicle with
the low tire pressures is the one you're driving? Wouldn't you want to be
warned?
The United States Department of Transportation (DOT) National Highway
Traffic Safety Administration (NHTSA) has developed a Federal Motor Vehicle
Safety Standard that requires the installation of tire pressure monitoring
systems (TPMS) that warn the driver when a tire is significantly
underinflated. The standard applies to passenger cars, trucks, multipurpose
passenger vehicles, and buses with a gross vehicle weight rating of 10,000
pounds or less, except those vehicles with dual wheels on an axle.
Maintaining the correct tire pressure for a vehicle is an important factor
in how much load its tires can safely carry. The correct pressure will carry
the weight without a problem. Too little tire pressure will eventually cause
catastrophic tire failure.
Tires aren't invincible. They are made of individual layers of fabric and
steel encased in rubber. If a tire is allowed to run low on air pressure,
the rubber is forced to stretch beyond the elastic limits of the fabric and
steel reinforcing cords. When this happens, the bond between the various
materials can weaken. If this is allowed to continue, it will eventually
break the bonds between the various materials and cause the tire to fail.
And even if the tire doesn't fail immediately, once a tire is weakened it
won't heal after being reinflated to the proper pressure. So if a tire has
been allowed to run nearly flat for a period of time, the tire should be
replaced, not simply repaired or reinflated.
Studies have shown that running tires with too little air pressure is not
uncommon. It's been estimated that about one out of every four vehicles on
the road is running on underinflated tires. This also means that one out of
every four drivers is needlessly sacrificing their vehicle's fuel economy
and handling, and reducing their tires' durability and tread life.
This has made tire pressure maintenance an important safety issue throughout
the automotive industry and caused the U.S. government to pass legislation
mandating tire pressure monitoring systems. The main purpose of these
systems is to warn the driver if their tires are losing air pressure,
leaving the tires underinflated and dangerous.
What types of systems are being used now? How do they work? Which works the
best?
The National Highway Traffic Safety Administration (NHTSA) provides vehicle
manufacturers options with which they can comply with the law. One option is
to install a direct tire pressure monitoring system that uses pressure
sensors located in each wheel to directly measure the pressure in each tire
and warns drivers when the air pressure in any of their tires drops at least
25% below the recommended cold tire inflation pressure identified on the
vehicle placard. Another option is to install an indirect tire pressure
monitoring system that would warn the driver when a single tire has lost at
least 25% of its inflation pressure compared to other tires on the vehicle.
While direct systems could offer more precise warning thresholds, indirect
systems cannot offer the same information or accuracy.
What's the Difference?

DIRECT VS. INDIRECT
Direct Systems...
attach a pressure sensor/transmitter to the vehicle's wheel inside the
tire's air chamber. An in-car receiver warns the driver instantly if the
pressure in any one tire falls below a pre-determined level.
Indirect Systems...
use the vehicle's antilock braking system's wheel speed sensors to compare
the rotational speed of one tire vs. the others. If one tire is low on
pressure, it will roll at a different number of revolutions per mile than
the other three tires, and alert the vehicle's onboard computer.

Direct Monitoring Systems
Direct tire pressure monitoring systems measure, identify and warn the
driver of low pressure. Because direct systems have a sensor in each wheel,
they generate accurate warnings and can alert the driver instantly if the
pressure in any one tire falls below a predetermined level due to rapid air
loss caused by a puncture. In addition, direct tire pressure monitoring
systems can detect gradual air loss over time. Some direct systems use
dashboard displays that provide the ability to check current tire pressures
from the driver's seat.
Direct systems attach a pressure sensor/transmitter to the vehicle's wheel
inside the tire's air chamber. Most Original Equipment and some aftermarket
systems attach their air pressure sensor/transmitter to special tire valves.
While the presence of a metal clamp-in valve typically identifies the
presence of a direct tire pressure monitoring system, special snap-in rubber
valves have also been used to support direct system sensors. The
transmitter's signal is broadcast to the in-car receiver and the information
is displayed to the driver.
Some aftermarket and Original Equipment direct monitoring systems attach the
sensor/transmitter to the wheel with an adjustable metal strap. These
sensors/transmitters and their straps only weigh a few ounces and allow
virtually universal application on car and light truck wheels. Since
standard snap-in rubber valves are still used for these applications, it is
important that the owners of these systems let their tire installer know
that the vehicle is equipped with a direct system banded to the wheel before
they change the tires.
The Tire Rack works with wheel manufacturers to develop aftermarket wheels
that accommodate direct tire pressure monitoring sensors/transmitters. This
results in our ability to offer a wider selection of aftermarket alloy wheel
styles that accept Original Equipment direct system components.
Additionally, The Tire Rack's fitment specialists have carefully determined
which aftermarket wheels will be compatible with the vehicle and system
installed for customers purchasing Tire & Wheel Packages or wheel upgrades.
Search results include notes regarding TPMS sensors and sensors can
confidently be purchased online with wheels.

Indirect Monitoring Systems
In the interest of providing a lower cost Original Equipment system,
indirect tire pressure monitoring systems were developed by vehicle
manufacturers wishing to comply with the law while minimizing development
time and cost. Indirect systems use the vehicle's antilock braking system's
wheel speed sensors to compare the rotational speed of one tire to that in
another position on the vehicle. If one tire is low on pressure, its
circumference changes enough to roll at a slightly different number of
revolutions per mile than the other three tires. Reading the same signal
used to support ABS systems, the vehicle manufacturers have programmed
another function into the vehicle's onboard computer to warn the driver when
a single tire is running at a reduced inflation pressure compared to the
others.
Unfortunately, indirect tire pressure monitoring systems have several
shortcomings. Indirect systems won't tell the drivers which tire is low on
pressure, and won't warn the driver if all four tires are losing pressure at
the same rate (as occurs during the fall and winter months when ambient
temperatures get colder). Additionally, our current experience with indirect
systems indicates that they can generate frequent false warnings. We have
found that false warnings may occur when the tires spin on wet, icy and
snow-covered roads. In these cases, the false alarms would train the driver
to disregard the tire pressure monitoring system's warnings, negating its
purpose completely.

Concerns
While The Tire Rack applauds the emphasis on maintaining appropriate tire
pressure and requiring a system that will warn the driver if low pressure is
detected, we are concerned about the percentage of underinflation that the
law permits before warning the driver. The driver of a passenger car that
calls for 35 psi may not be warned about tire pressure loss until it drops
to 26 psi depending on the type of monitoring system used. Under the same
circumstances, a driver of a light truck that calls for 80 psi won't be
warned until just 60 psi remains. In both of these cases, significant load
capacity has been sacrificed before the driver is warned.
The only way to overcome this obstacle would be to fit significantly
oversized tires to every new vehicle that could compensate for a 25% loss in
tire pressure before becoming overloaded. Unfortunately, these larger tires
would add to gross vehicle weight, generate more rolling resistance and
increase the vehicle's aerodynamic drag. This would result in a loss of fuel
economy and increased gasoline consumption in direct contrast to the
government's Corporate Average Fuel Economy (CAFE) requirements for cars and
light trucks.
While the legislation is well intended, we feel that direct tire pressure
monitoring systems are the better means to warn the driver of low tire
pressure before inconvenience becomes calamity. Additionally, we are
concerned that the drivers of vehicles equipped with any tire pressure
monitoring system will become over confident in the capabilities of their
system and will be even less likely to confirm their vehicle's cold tire
pressure with a pressure gauge at least once a month and before long trips.