Monitoring Your Tire Pressure

How Does TPMS Work and Why Do You Need It?

Some OEM tire pressure monitor systems display the pressure for each tire on the dash. Photo © AJ Batac

What is a Tire Pressure Monitoring System?

Tire pressure monitoring systems (TPMS) continuously check the pressure in the tires of a vehicle and report that information to the driver. Most of these systems measure the pressure directly, but some infer pressure from observing factors like the rotational speed of the tires.

The first tire pressure monitoring systems appeared in the 1980s, but the technology didn’t become ubiquitous until much later. Adoption of the technology in the United States was spurred by the TREAD Act of 2000, which required all light motor vehicles in the US to have some form of TPMS by 2007.

What’s the Point of Tire Pressure Monitoring?

Tire pressure often affects handling characteristics, which is the primary reasoning that governments have used to legislate the use of these systems. Underinflated tires can contribute to increased braking distances, poor lateral stability, and other issues. If a tire is sufficiently low on air, it can even overheat and fail catastrophically. When that happens at high speeds, the results can be devastating.

There is also an economic reasoning behind tire pressure monitoring that should appeal to any budget-minded car owner. Underinflation can have an adverse affect on gas mileage and tread wear, so keeping your tires properly inflated can save you money over time. If your tires are underinflated by 10 percent, you’ll typically experience about a 1 percent reduction in fuel efficiency. That might not seem like a lot, but it has a cumulative effect.

How Does Tire Pressure Monitoring Work?

Most tire pressure monitoring systems use physical pressure sensors, battery-powered transmitters, and a central receiver unit. Each tire has its own pressure sensor, and the battery-powered transmitters report the individual pressures to the receiver. That information is then processed and presented to the driver. In most cases, the system is designed to alert the driver if any of the tire pressures drop below a certain threshold.

The other method of monitoring tire pressure is sometimes referred to as an indirect tire pressure monitoring system (iTPMS). These systems aren't actual tire pressure gauges and don’t measure tire pressure directly, so they don’t have battery-powered transmitters that require periodic replacement. Instead, indirect measurement systems look at factors like the rotational speed of the wheels. Since tires that are low on pressure have smaller diameters than fully inflated tires, it’s possible for these systems to infer when tire pressure needs to be adjusted.

What Are the Different Types of Systems?

The two main types of tire pressure monitoring technology are TPMS and iTPMS. However, there are also two main types of sensors used by tire pressure monitoring systems. The main type of TPMS uses sensors that are built into the valve stems of each tire. Each valve stem assembly has a sensor, transmitter, and battery built into it. These components are hidden inside the wheels, and they can only be accessed by removing the tire. Most OEMs use this type of TPMS, but there are a few downsides. The sensors are typically very expensive, and they tend to be somewhat fragile.

The other type of TPMS uses sensors that are built into the valve stem caps. Each cap contains a sensor, transmitter, and battery just like the in-wheel versions. However, this type can be installed without demounting the tires. The primary disadvantage is that the sensors are readily identifiable, which makes them vulnerable to theft. Both types of TPMS also have other advantages and disadvantages.

Can I Get Tire Pressure Monitoring on My Vehicle?

If you buy a new car in the United States or European Union, it already has some type of TPMS. All vehicles in the US have had them since 2007, and the EU instituted a mandate in 2012. If your vehicle is older than that, it is possible to retrofit it with an aftermarket system.

Both valve stem and cap systems are available from the aftermarket, so you have your choice of systems. Valve stem sensors tend to be more expensive, and they require a trip to your mechanic for installation. Most shops charge a nominal fee to demount and mount tires, but the actual installation of the sensors is typically free. That’s due to the fact that installing a valve stem tire pressure sensor is no more complicated than installing a regular valve stem. If you’re already buying new tires, most shops will install sensors at that time for no extra labor charge.

If you don’t want to take your car to a tire store or repair shop to have sensors installed, then you can buy an aftermarket TPMS that uses cap sensors. These systems can be installed by simply replacing your existing valve stem caps with the sensors from a TPMS kit. Most kits also have a 12 volt adapter that you can plug into your cigarette lighter or accessory socket.