[NYVette]

The advantage to nitrogen in automotive tires is that it is a thermally stable inert gas. This means that once inflated to a particular PSI the tire will remain at that pressure hot or cold. That is why racecars use nitrogen...race tires get very hot and are pressure sensitive for proper performance. The FAA...An aircraft has air put in the tires while on the ground at ambient temperature/pressure It then climes to high altitude with corresponding cold and low ambient pressure so the inflation pressure would change. If the plane were to land with cold tires, the tires would be underinflated because the air in them has contracted reducing pressure until they are back to the original temperature. AAA (the auto club)states that something like one in five people regularly check tire inflation pressures. If you are one of these people and almost never check tire pressure, and you switch to nitrogen in your tires, then you might see an increase in MPG. This is because the tire stays at the recommended pressure instead of slowly loosing pressure which increases rolling resistance of the tires and subsequently causes more fuel consumption...and shorter tire life. So, maintain the recommended pressure all is well, even with air in the tires. As JohnN said, "nitrogen has larger molecules". It takes far longer for nitrogen to leak out of a tire than it does for air. On a molecular level rubber is porous. Nitrogen's larger molecules have a difficult time slipping through these microscopic holes but the other gases that make up out atmosphere can more easily get through. By the way....air is 78% nitrogen.
The fat low pressure tires used on golf carts definitely suck up energy. However these same tires keep you and the cart from sinking into soft turf because of a large contact surface relative to vehicle weight. If used only as a street transport a golf cart would have increased range if tires with lower rolling resistance were used.