How does measured tire pressure vary with altitude? If I inflate a tire to the recommended cold pressure of 30 PSI in Yellowstone (elevation 8000 feet), then drive to Big Timber (elevation 4000 feet) and measure the pressure the next morning at 26 PSI, does this mean I have a slow leak, or is this due to the change in atmospheric pressure?
I won't go into a whole lot of detail, but read this article here. http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=167
Pay special attention to this part as it pertains directly to your question.
Since a tire mounted on a wheel essentially establishes a flexible airtight (at least in the short term) pressure chamber in which the tire is shaped and reinforced by internal cords, it retains the same volume of air molecules regardless of its elevation above sea level. However, if tire inflation were set with a tire pressure gauge at sea level (where the atmospheric pressure of 14.7 pounds per square inch is used as ambient atmospheric pressure by the gauge), the same tire pressure gauge would indicate the pressure has increased at higher elevations where the ambient atmospheric pressure is lower. Those measured at the 5,000-foot level (where an atmospheric pressure of only 12.2 pounds per square inch is the ambient pressure) would indicate about 2-3 psi higher than at sea level. On the other hand, traveling from a high altitude location to sea level would result in an apparent loss of pressure of about 2-3 psi.
With that being said, your tires should be fine. If you're worried about it, just inflate your tires to the recommended pressure at the altitude that you spend most of your time at. The differences in altitude will cause varying readings on your gauge, but the pressure inside of the tire itself is contained and has a constant volume.