Virtually all new vehicles now come with a Tire-Pressure Monitoring System as standard equipment. Such a system has been required in the U.S. since September 2007, and other countries and regions--including Canada, the European Union, South Korea, Russia, and others--have followed suit or are in the process of doing so.
The systems work by reading real-time data transmitted by small, battery-powered sensors mounted on the inside of each wheel rim. They send data to nearby low-frequency receivers, after which it's processed by the car's control system and displayed on the instrument panel. Some vehicles have displays like the one above, which show a numeric tire-pressure reading for each tire. At minimum, a warning light showing that tire pressure is low will illuminate when that is the case.
Because the tire-pressure monitoring system will fail if the sensor batteries run down, cars use a variety of tactics to conserve battery life. In general, the sensors will not transmit when the wheel is not in motion. This prevents the spare tire, for instance, from sending signals indicating its pressure. When batteries do finally give up the ghost, the entire sensor and battery unit generally has to be replaced--which requires removing the wheel from the car, removing the tire from the wheel, and removing and installing the unit.
The rule requiring tire-pressure monitoring systems was passed by the U.S. National Highway Traffic Safety Administration at the behest of Congress, following investigations into a series of accidents that killed more than 100 people in the late 1900s. All the incidents involved failures of Firestone rear tires on Ford Explorer sport-utility vehicles. Under-inflated tires in heavily loaded Explorers was deemed a contributing factor in the failures that led to the accidents.
But very few drivers regularly check their tire pressure, so the new systems were considered a safety feature likely to prevent injuries and save lives. Drivers who keep their tires properly inflated also save fuel and extend the life of those tires as well.