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Tire Guide

Checking tire pressure is key to safety

Published August 24, 2013

How often do you check the tire pressures on your car or truck? If you’re like most drivers, probably not often enough.

According to a recent study cited by Transport Canada, about half the vehicles on Canadian roads have at least one tire that is either over- or under-inflated by more than 10 per cent.

In fact, one in 10 vehicles surveyed had at least one tire under-inflated by 20 per cent or more — enough to represent a real safety issue.

Previously, this column has addressed how under-inflated tires can negatively affect fuel consumption. But the consequences of under-inflation can be much greater than just some extra fuel cost.

According to the U.S. National Highway Traffic Safety Administration (NHTSA), the leading cause of tire-failure is under-inflation.

Vehicles with under-inflated tires are three times more likely to be involved in a collision linked to tire problems than those with tires inflated to the correct pressure, NHTSA says.

Related: How old is too old for tires?

Transport Canada agrees: “Operating an under-inflated or overloaded tire at highway speeds on a warm summer day is a recipe for tire failure.”

People tend to equate loss of tire pressure with a puncture, but there are other causes as well. It can result from a gradual slow leakage through a tire’s valve stem or through the membranes of the tire itself, as well as from changes in temperature.

Typically, tire pressure can drop about 7 kPa (1 psi) per month through inherent sources of leakage. It also drops about 7 kPa (1 psi) for each 5C reduction in ambient temperature.

Driving on an under-inflated tire can negatively affect your vehicle’s handling, as well as cause premature and uneven tread wear. It can also increase stress on the carcass itself, through flexing and overheating, potentially leading to catastrophic structural failure.

For all those reasons, in 2005, regulators in the U.S. established a safety standard requiring mandatory installation of a tire pressure monitoring system (TPMS) on new light-duty cars and trucks. That requirement took effect with the 2007 model year.

It specifies that a TPMS system must warn drivers if the pressure in any one tire falls 25 per cent or more below the vehicle manufacturer’s recommended cold tire inflation pressure (as specified in the owner’s manual and on a label affixed to the vehicle, usually on the driver’s door post.)

The regulation allows manufacturers some latitude in the sophistication of their TPMS systems.

At minimum, they must trigger a warning light of a specified design on the instrument panel if the pressure in one or more tire falls below the designated level. They must also signal a malfunction in the system itself.

It is not mandatory that the tire with low pressure be identified but only that the driver be warned that one or more tires needs attention.

Some systems go much further, to the point of displaying actual individual tire pressure, usually on a menu display so the driver can select that information at any time.

Although several methods have been developed for monitoring tire pressure, the most common is a sensor mounted on the wheel, inside the tire, that transmits data to a receiver via RF (radio frequency) waves.

Given that Canada has effectively harmonized its safety standards with those of the U.S., you might expect all new vehicles sold in this country to be equipped with TPMS.

Many are. But don’t count on it.

That regulation is one of the few exceptions to our government’s policy of harmonization. TPMS is not required in Canada.

Why? According to Transport Canada, based on its research and collision-investigation programs, it has not identified a pattern of collisions caused conclusively by tire failure.

You might also speculate that consumer complaints about false warnings and malfunctions had something to do with the Canadian decision — particularly given the broad range of temperature fluctuations experienced here.

The common practice of switching wheels and tires between seasons may also have been a consideration.

As a result, your vehicle may or may not be equipped with TPMS. If you’re not sure, check your owner’s manual to find out.

Either way, it’s still a good practice to check and adjust tire pressures at least once a month and before any long trip.

Set the pressures to the levels recommended by the vehicle manufacturer — not the maximum pressure figure found on the tire sidewall, which is a very different thing.

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