COLUMN: Making sense of a pressure-packed situation

The main issues, according to Goodyear, seem to be handling and performance.

According to the tire maker, "tires inflated over the vehicle manufacturer-recommended inflation pressure:

• "Will be stiffer than the manufacturer intended, which can result in reduced ride comfort, due to less ability to absorb road irregularities, and (have) greater susceptibility to tire/suspension damage because of harder impacts with pot holes and hazards;

• "Will have a smaller contact patch with the road, meaning less traction; and

• "Can be prone to fast center line wear due to reduced deflection in the contact patch area."

Goodyear said the more a tire is overinflated, the more these conditions will be amplified. For example, a tire with a vehicle manufacturer-recommended inflation pressure of 30 psi will be 17 percent overinflated at 35 psi, a condition Goodyear described as "significant." At 42 psi, or plus 12 psi in this example, the tires would be 40 percent overinflated.

Asked why proper inflation is so important, the tire maker noted that it's the inflation pressure in the tire that supports the vehicle's load, and "both over- and underinflated tires can lead to issues of uneven wear, accelerated wear, increased susceptibility to impact damage and sub-optimized handling and stopping power."

Goodyear also noted that "because a given tire size might be fitted by various vehicle manufacturers on different cars — all with different loads and driving characteristics — the recommended tire inflation pressure for any specific car is specified by the vehicle manufacturer and always should be found on the car."

In fact, this information can be found in several places in the vehicle, Goodyear said, including the owner's manual, and/or on the label inside the driver's door-side jamb, or inside the glove box or on the inside of the fuel door.

So why is there a maximum inflation pressure for tires any way, especially since vehicle manufacturers usually recommend an inflation pressure that's less than the tire's maximum? Aren't these different inflation numbers just a way to cause confusion?

Goodyear explained that the "U.S. Federal Motor Vehicle Safety Standards require tire makers to design tires to support one of three maximum inflation pressures: 240 kPa (35psi); 300 kPa (44 psi) or 350 kPa (51 psi) for light-load or standard tires, or 280 kPa (41 psi) and 340 kPa (50 psi) for exta-load tires. The maximum inflation pressure must be marked on the sidewall in both kPa (kilopascal) and psi, with kPa/6.895 equaling one psi."

Each tire size also is governed by an industry-standardized load table that lists the maximum pressure for a given tire size to support various loads up to the maximum for that size, the tire maker said.

"In cases where the inflation pressure required to support the tire size max load is less than the max inflation pressure for the tire, the auto manufacturer uses the difference to optimize the tire for handling balance and response," Goodyear said.

So why were the tires on the Altimas I rented aired up higher than the vehicle manufacturer recommended?

After talking with the car rental company and my local Nissan dealership, I think I have a clue. Both vehicles I drove were nearly new, with only several hundred miles on them.

The Nissan service manager said his dealership often airs the tires on vehicles it has sitting on its lots up to 45 psi to avoid flat spotting them.

Once the new cars are put into service, the tires usually are checked, and the inflation pressures are reset to the vehicle-makers' specs. That is unless someone forgot to do this.

Which brings me to one additional question: How often do tire dealerships see overinflated tires in their customers vehicles? Is this a common occurrence or rare? Send me an email at [email protected] and let me know.

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Mr. Zielasko is editorial director and VP/publisher of Tire Business.