I work for a car dealer that sells nitrogen in tires. It is not a cure all but it does definalely help. We put some in motorcycles also. The nitrogen molecule is larger and does not leak out nearly as quickly. Also doesn't fluctuate pressure with temprature as much. Also much dryer inside the tire. Doesn't rust the rim. We sell it for $19.95
I use a special proprietary mixture of about 78% nitrogen, and about 21% oxygen, and the rest mostly argon, processed through a miracle known as a line-dryer.