[Wally15]

The primary reason to put nitrogen in your tires is because nitrogen is an inert gas, it doesn't really react with anything. The 21% oxygen in "air" is a reactive gas, actually oxygen is a corrosive. Think rust! By eliminating the O2 inside the tire, you reduce the chances of something happening to weaken the tire or rim internally, especially a steel rim. Hopefully the tire will keep the same "stuff", whether it's nitrogen or air, inside for 5-6 years.
Oh, the difference is size between an O2 and an N2 molecule is
only about 0.3 times 10 to the -10th meters (0.00000000003 meters). I looked it up.