By the time you can visually detect low air pressure in a tire, it's already too late. You have likely shortened its life, and could potentially cause a tire failure at an untimely moment.

"Contractors can identify if a tire is flat," says Wayne Birkenholz, manager of global field engineering, Firestone. "But relying on visual inspection is a matter of degrees."

For example, let's say the optimum inflation pressure for a tire is 100 psi. "Can you tell when it's 20 lbs. underinflated?" he asks. "The tire may not look low at that level. You can tell that it has air in it, but you can't tell if it has 70 lbs. or 100 lbs. But that much difference could certainly affect performance."

DOT regulations indicate that a tire is considered flat at 20% below the recommended inflation pressure. However, Doug Jones, Michelin customer engineering support manager for North America, notes that detrimental affects can be experienced at much less than that. "At just +/- 5% above or below the recommended inflation pressure, you can start sacrificing tire life," he says. "A tire that is underinflated by 10% will experience about a 10% decrease in service life. A tire that is overinflated by 10% will experience about a 5% decrease in service life."

The tire and wheel assembly is simply a container that holds air. How much load you can carry with that container is dependent on the size of the tire (i.e., how large is the air cavity) and the inflation pressure. For a given load, if you reduce the air cavity, you have to increase air pressure. Conversely, if you increase the cavity, you can decrease the inflation pressure. "For a given load, a larger tire requires less inflation pressure," says Jones. "A skid-steer loader tire is a relatively small tire with heavy loads, so we have to run relatively high inflation pressures."