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."
Contractors may be tempted to overinflate tires, since this has a positive affect on fuel economy in tires used on the road, says Kevin Lutz, Michelin. But that fuel savings comes at a cost, especially for tires used off road.
"Overinflation will give you better fuel efficiency on the road," says Lutz. "But overall, it will cost in lost productivity. That is especially apparent in equipment such as a backhoe. It costs in productivity because it takes you longer to the do the work. You can spend an extra 10% to 15% more time doing the job, and that costs in fuel."
Plus, overinflation of both on- and off-road tires makes them more susceptible to punctures. "If you hit a rock, you're more likely to break the body plies," says Birkenholz. "It can also affect the wear pattern of the tire - you can have more rapid, less uniform wear. It's harmful because it changes the way the tread goes into the footprint."
The telltale sign of an overinflated tire is wear in the center of the tread. The excess inflation pressure rounds out the tire and reduces the size of the footprint. That in turn sacrifices flotation and traction, and increases the possibility of getting stuck.
"With overinflation, you affect your ability to get through soft, wet soils," says Lutz. "More footprint on the ground gives you better traction. If you shrink that by overinflating the tire, your tractive effort is less. You may not be able to fill your bucket as full or it may take longer to fill it."
Underinflation tends to be more common in off-road vehicles, because it can give you the larger footprint needed for extra tractive effort. But like overinflation, it can wreak havoc on tire life and severely sacrifice durability.
Underinflation shows up as excess wear in the shoulders at the outside edges, because the center of the tread tends to concave. "It doesn't contact the footprint area," says Birkenholz. "With proper inflation, you get good, uniform contact pressure across the face of the tread for nice, even wear."