Most newer vehicles have a built in tire pressure monitoring system (TPMS), which continuously monitors the air pressure of all four tires. In most cases the TPMS alerts drivers through a warning light on the dashboard when tire pressure falls below the recommended preset limit. There are two different TPMS types – direct and indirect – each works in different ways.

A direct TPMS employs a pressure sensor on each tire, typically located on the valve stem or band mounted. Direct system sensors are powered by separate lithium batteries, which ultimately die. If the vehicle has a direct system, its regularly scheduled maintenance will include a check to make sure those batteries are working properly.

Indirect TPMS technology uses a different approach, with monitoring based on a calculation of factors, starting with the fact that the over-all diameter of a tire is smaller when it is underinflated. A smaller wheel has to spin faster to keep up with the other three. Wheel speed sensors located at each wheel position pick up on a low or fast-spinning tire by comparing the rotational speed of each wheel with the average speed of all four wheels. The indirect system does not require servicing, it is not without problems. For example, if all four tires are underinflated, the indirect TPMS may not detect a problem, since all four wheels are spinning faster, with no discernible difference.

As great as new TPMS technology is, it is not time to get rid of the classic tire gauge just yet. Like any electronic feature on your vehicle, the TPMS is not entirely fail-safe. Regular manual tire pressure checks will give you the added measure of confidence that your TPMS is working properly. Also, doing that regular manual check will remind you to take a close look at your tire tread, which is something you should be inspecting on a regular basis as well.