How Do Tire Pressure Monitoring Systems Work? A Look At The Tech Behind TPMS

Your car's tires are a vital element of your car's overall safety, and absolutely must be in ideal working order to ensure a safe driving experience. However, while it's easy enough to clock a completely flat tire at a glance, if one or more of your tires is only slightly below the ideal PSI, you probably wouldn't be able to tell. You could be driving with improperly-inflated tires and not realize until the pressure difference starts negatively affecting your experience. 

This is why it's vital that all cars are equipped with a tire pressure monitoring system, or TPMS. It's thanks to your car's TPMS that the little light letting you know your tires aren't properly inflated winks on, which in turn lets you know it's time for a trip to the gas station air pump. While these systems all serve the same purpose, some of them go about it in slightly different ways using more advanced monitoring technologies.

Broadly speaking, a TPMS is constantly monitoring the pressure inside of your car's four tires. When one or more of the tires is exhibiting an improper air pressure, that information is relayed to your car's computer. This manifests as a warning light on your dashboard, which will remain illuminated until the correct pressure is detected, i.e. once you've pumped the right amount of air into the tire based on your car's pressure placard.

Interestingly, TPMS technology existed in a prototypical state as far back as the 1970s, and was gradually implemented onto personal vehicles through the 1980s. However, these systems weren't actually mandated on vehicles until the year 2000, when a series of severe crashes caused by underinflated tires prompted the passing of the Tire Recall Enhancement, Accountability, and Documentation (TREAD) Act by the United States Congress. From 2007 onward, all passenger vehicles weighing less than 10,000 pounds needed a TPMS, full stop.

Tire pressure monitoring systems are broken up into two general categories, indirect monitoring and direct monitoring. From a driver's perspective, both of these systems do the same thing: turn on that little warning light when something's gone wrong inside your tires. The actual tech behind them, though, is quite different.

In an indirect TPMS, the focus is on the rotation of your car's wheels, rather than the actual pressure in the tire. A small sensor located inside of your car's wheels is constantly monitoring the speed at which they rotate. When your tires are properly inflated, the wheels are supposed to spin at a consistent rate. If your tire's inflation is lacking, though, the wheel's overall size is reduced, which causes it to spin faster than normal. Should this happen, the sensor in the wheel takes notice and sends a warning to the car's central computer, prompting a warning on the dashboard.

Despite their indirect natures, the information gleaned from an indirect TPMS is generally pretty precise and reliable. The only real foible is that, after you reinflate your tire, you need to reset the system. If you don't, it'll assume that your tires have just spontaneously increased in size, which it'll read as a hazard and flash the warning light at you.

A direct TPMS works a little more like you'd expect, with its sensors located right inside of the tires themselves. This sensor checks the ambient pressure inside of the tire, as well as relevant data like temperature. Rather than a direct connection to your car, these kinds of sensors will usually relay information to the central computer via a small battery-powered radio transmitter. That data can be used to get a detailed readout of how your tire is doing, including its current pressure and handling behavior. Naturally, if the pressure is well beyond safe values, the warning will go out from the sensor and the dashboard light turns on.

One major weakness of a direct TPMS is that, since the sensor isn't directly connected to the car, its internal battery is the only source of power it has. Once that battery starts to falter, the efficacy of the sensor may start to drop off before eventually failing entirely. Sensors can last anywhere from 5-10 years, with a replacement being necessary upon failure. Some sensors automatically power down when the tire isn't spinning, which helps to preserve battery life, though that does mean you won't get any data until your cars is at speed.