[NuRAVer]

Over time your tires will loose air regardless of driving conditions unless they are replenished with air. Tires are not perfectly impermeable and air molecules actually slowly permeate through the tire material. This is why a spare tire can loose pressure even when there is no tire, wheel, or valve stem defects.

I view the tire pressure monitoring system as for safety back-up, not a maintenance reminder. It is no subsitute for regularly checking pressure on your tires using a pressure gage (You can get a good one for less than 5 bucks at any auto store). Recommend doing this on a weekly basis. By keeping the tire pressures at the specified pressures rather than waiting for the tire pressure light to come on, you will get better gas mileage and reduce tire wear. During the regular tire pressure checks, you will also be able to do a visual check of the tires for any obvious damage and monitor tire wear.