@Tornado - Following is the composition of air -
Air is a mixture of 78.03% nitrogen, 20.99% oxygen, 0.94% argon, 0.03% carbon dioxide, 0.01% hydrogen, 0.00123% Neon, 0.0004% helium, 0.00005% krypton and 0.000006% xenon.
@Sam Dinkar - you are right. No significant difference between filling nitrogen or air as air is almost 80% nitrogen, oxygen is not significantly heavier than nitrogen considering the volume of gas held in tyres.
its just another automobile fad to fool innocent people.