Why Check Spare Tire Air Pressure?

What happens to rubber balloons in the days after a party? The air pressure inside them escapes through the rubber and the balloons go flat. And while air also escapes through the rubber of tires for cars and trucks, today's more robust tubeless tire designs experience air pressure losses of only about one psi per month. Hence, the industry recommendations to check tire pressures at least on a monthly basis.

As a test, we checked, recorded and reset the pressures in the tires on the students' vehicles used for one of our Tire Rack Street Survival Teen Driving Schools held at our South Bend headquarters. In addition to simply resetting the tire pressures of the four tires on the ground, we also checked the air pressure of the spare tires as well.

Cars and light trucks can be equipped with several types of spare tires, ranging from full-size spares that are identical to the other tires on the vehicle to narrower and often shorter diameter mini-spares. Full-size spares typically specify the same pressures as the other tires on the vehicle while the smaller, temporary-use mini-spares require 60 psi to develop enough load capacity to carry the vehicle.

What did we find? Surprising results that could lead to catastrophic consequences if these spares were put into service.

A few examples include:

• Full-size spare specifying 29 psi inflated to 22 psi (25% underinflated)
• Mini-spare specifying 60 psi inflated to 17 psi (70% underinflated)
• Full-size spare specifying 32 psi inflated to 16 psi (50% underinflated)

A Spare Tire is no Spare Without Enough Air

Since tires require inflation pressure to provide load capacity, if any of these tires had been placed into service without increasing their inflation pressures, they would have been underinflated, overloaded and a catastrophic failure waiting to happen. Even if they were driven on while underinflated for a short period of time at relatively low speeds, the bond between their rubber compounds and reinforcing cords would have been weakened forever.

While maintaining correct tire pressures is a common problem, these results emphasize how little importance drivers place on maintaining them and the spare. Tire Rack recommends that all drivers check and adjust cold tire inflation pressures of all tires, including the spare, at least once a month or before taking any extended highway trips.

Since tires don't just lose pressure during National Tire Safety Week, maybe the event needs to be held all year long and become simply an ongoing initiative for National Tire Safety Awareness.