One positive outcome of the Ford Explorer-Firestone debacle -- in which the tires or vehicle were blamed for a number of incidents, including some deadly crashes -- was an awareness of the need to maintain proper tire pressure.

While the vast majority of those crashes, if not all, were caused by overloading, improper maintenance or under-inflation -- or a combination of those -- the result has been intense pressure on the tire and auto industries to do everything possible to avoid a repeat.

Shortly after the events gained international prominence, the National Highway Traffic Safety Administration in Washington issued requirements for tire pressure monitors. The hastily prepared regulation was subsequently shot down by the U.S. Supreme Court over issues regarding wording, causing the NHTSA to go back to the drawing board.

Last September, it released a reworded proposal to require all passenger vehicle to be equipped with warning lights, telling drivers when tire pressure is too low.

If approved, the new regulation will require manufacturers to install a four-tire pressure monitoring system on all light vehicles by 2007.

The new wording also requires the system to issue a warning when it is not working properly. Fifty per cent of vehicles are required to have them in model-year 2005, and 90 per cent in 2006.

There is still some debate about the wording, centred around the suggested flat rate of 25 per cent under-inflation to trigger the alarm. The Rubber Manufacturers Association, a trade group representing tire makers, argues that the pressure monitors might actually increase the risk of tire failure by not providing a timely warning.

It says a 25-per-cent drop in pressure may be insufficient to safely carry a fully loaded vehicle and that motorists could travel thousands of kilometres on under-inflated tires. Under-inflation causes heat buildup that results in hidden damage and subsequent tread separation and other critical issues leading to tire failure.

But nobody is arguing about the benefit of monitoring tire pressure. Expected to add about $90 to the cost of a vehicle, the systems might well recover that costs in increased fuel mileage and reduce maintenance through longer tire life.

Tire pressure monitors are not exactly new.

Some manufacturers started installing systems about 15 years ago. GM, for example, began doing so in 1987 and has more than two million vehicles on the road with this technology.

These early tire "indirect" pressure monitor systems generally worked by measuring vehicle ride or axle height. The sensors used by the ABS system report on a difference in rotational speed between wheels. A smaller tire or rolling radius due to lower pressure will rotate faster. This increased rate of rotation results in a warning on the instrument panel.

The new generation of "direct" systems measure and monitor the pressure in each tire through small devices actually installed in the tire. These systems use a tiny transmitter that sends a signal to a receiver connected to an alarm system.

The older systems will not be allowed under the new regulations since they cannot tell whether all four tires, two tires on the same end of the vehicle or two on the same side, are under-inflated.

In both cases the systems send a signal to a set of lights in the instrument panel.

The least elaborate ones simply report a problem and leave it to you to figure out which tire. The more modern and elaborate ones will provide a readout of the actual pressure in each tire.

Systems are available that can be retrofitted to current and older vehicles. They start at about $250, plus installation.

Some include a display that gives both pressure and temperature for each tire. Most mount inside the tire, on the wheel or rim, requiring the tire to be removed.

Some incorporate the sensor in the valve stem.