Originally Posted by ChrisSlicks

My wheels (on my car) sat all winter and didn't lose any pressure at all with regular air when measured at equal temperature. According to my "tire guy" most of the pressure loss occurs as the rims get a few years older and oxidation occurs on the surface of the aluminum preventing a perfect seal. There are various methods to fix this during a tire swap, but only the good shops seem to bother. Of course there are plenty of other ways to lose air pressure such as leaky valve cores, hidden punctures etc. Also note that using a tank air compressor with water trap filter will give better results than a portable tankless compressor.