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.

Here’s two types of Tire Pressure Monitoring Systems in the market: Direct Monitoring Systems and Indirect Monitoring Systems

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.