Air Pressure, Time Fluctuations(Lea en español)
Your tires support the weight of your vehicle, right? Well they don't! It's the air pressure inside them that actually supports the weight. Maintaining sufficient air pressure is required if your tires are to provide all of the handling, traction and durability of which they are capable.
However, you can't set tire pressure...and then forget about it! Tire pressure has to be checked periodically to assure that the influences of time, changes in ambient temperatures or small tread punctures have not caused it to drop.
While tires appear solid, the molecular structure of the rubber used to manufacturer them actually looks like strands of cooked spaghetti stuck together. These molecular strands are stretched and returned to their relaxed state every time the tire rolls (about 800 times every mile).
Since typical tire pressures range from 30 to 35 psi for cars (with light truck tire pressures often higher), there is a constant force trying to push the air through the tire. This allows some of the air to escape (called permeation) right through the microscopic spaces between the rubber molecules. And somewhat like a rubber balloon, the air will eventually escape if it is not replenished.
All things being equal, a tire's inflation pressure will go down by about 1 psi every month. This means that if air isn't added for two to three months, the tire's inflation pressures will probably be 2 to 3 psi low.
Tires are normally inflated with air (a combination of gasses comprised of about 78% nitrogen (N2), 21% oxygen (O2) and 1% argon (Ar) along with traces of other gasses) from the local gas station. Unfortunately, using air permits moisture and the amount of water vapor in the air varies from place to place, time of the year and due to weather conditions. While air is all around us, finding a convenient source of compressed air is becoming difficult, and finding a source of "dry," vapor free compressed air is even more difficult.
In order to help maintain more constant tire pressures we should check them more frequently. Once a month and before trips is the minimum, once a week is preferred. This will allow us to refill lost pressure that escapes over time, as well as discover any pressure losses due to slow leaks caused by minor punctures before significant pressure is lost and the tire's internal structure is damaged.