As discussed in some of our other articles on inflation pressure, tires must be properly inflated to achieve maximum handling, traction, and durability as designed by the tire manufacturer. It is the air pressure that supports the weight of your vehicle, not the tire itself. Tire pressure should be monitored frequently for safe driving and optimal tire performance.
Tire pressure changes over time. Think of how balloons begin to lose air after a few days...the same thing occurs in tires. The air pressure inside the tire causes very small amounts of air to escape through the walls of the tire over time. Even though the tires are fairly airtight, there are still microscopic pores in the tire materials that let air out. This effect is called permeation.
Generally speaking, assuming no other air loss (such as a puncture or pin-hole leak), a tire inflated between 30-35 psi will lose approximately 1 psi per month. This means that if you neglect to add air and maintain proper inflation pressure for a few months, your tires could be 2-3 psi under-inflated.
The bottom line is that tire inflation pressure should be checked monthly. This will help to ensure that your tires are achieving their optimal traction, stability, and durability. In addition, by checking your tire inflation pressure monthly, you may notice if you have additional leakage due to a puncture or other small leak. It is much better to detect a problem early before the tire is damaged due to excessively low pressure.