A GUIDE TO TIRE PRESSURE MONITORING SYSTEMS (TPMS)
The federal government has mandated that all new passenger cars, multipurpose vehicles, trucks & buses weighing 10,000 pounds or less be equipped with tire-pressure monitoring systems (TPMS). The systems must indicate when one or more tires have lost 25% of their recommended inflation pressure, allowing consumers to avoid the resulting unsafe driving conditions. The rule is being phased in, with 20% of the affected vehicles required to comply by model-year 2006, 70% by 2007 and 100% by model-year 2008.
Generally speaking, there are 2 different types of tire-pressure monitoring systems. Indirect systems have been around for more than a decade. These devices use the vehicle's anti-lock braking system to monitor each wheel's rotational speed. As a tire loses pressure, its rolling radius decreases and its rotational speed increases. A malfunction indicator light (MIL) is triggered when one or more tires' rotational speed reaches a predetermined threshold in comparison to the other tires. In a direct system, each tire is equipped with a sensor that measures actual pressure. The measurements are then sent via a wireless radio-frequency transmitter to a receiver, which analyzes the signal and reports the results on a monitor mounted within the cabin of the vehicle. An under-inflation state in any tire activates an alarm.
A federal court in 2003 found that indirect system was not as reliable since it could fail if all four tires were under-inflated or if two tires on the same axle were under-inflated. NHTSA published a new rule in April 2005 requiring that the system be able to detect a loss of air pressure in each of the four tires.
The ability to reprogram the systems was another disputed topic. SEMA requested that the systems be reprogrammable in order to accomodate alternative tires with different pressure thresholds. If the car comes with tires inflated at 32 pounds, the warning light will come on if a tire drops to 24 pounds, which is 25% of 32. However, if the car owner then installs low-profile tires with a recommended pressure of 40, the warning light should come on at 30 pounds. But unless the TPMS is reprogrammed, it won't trigger until 24. That means the tire is now 40% under inflated, which would defeat the purpose of the rule and give the driver a false sense of security.
NHTSA did not want to mandate reprogramming. The agency believes that it will be a voluntary feature of TPMS in the marketplace and is unaware of any problems that would prevent reprogramming. Such problems are unknown at this stage, since the systems are just now coming into the marketplace in large numbers.
Once consumer get used to seeing something on their new vehicles, they want it on their old vehicles as well. Once they start using TPMS - esp. aftermarket model with its ability to display current pressure at any point and to warn of low tire pressure - they feel naked when they're driving a vehicle without it.
A consumers must understand that a TPMS system will eventually pay for itself through reduced tire wear and increased fuel economy. Right now, DOT says that we waste about 4.2 million gallons of fuel a day due to low tire pressure. That's a little over 2 billion gallons a year, and we're sitting with a U.S. reserve of about 900 million gallons, so we're wasting over 2 times that. The tire industry and the government tell us that somewhere around 32% of all tires are running at least 30% low. That means that those tires are dangerous on the road, which affects insurance, vehicles and health.
The Insurance Institute for Highway Safety said that 200,000 accidents last year (2005) were caused due to blowouts. In addition, you wear a tire out roughly 10% faster for every 10% it's under inflated. So if we're 30% low on 32% of the tires, that's a lot of wasted tires and a lot of wasted resources that will be used to replace them. We like to think that, aside from providing safety and savings in fuel and tire wear, we're also providing people with an opportunity to help environmentally.
Reasons to always keep your tires properly inflated (i.e. prevent under-inflated & over-inflated tires):