OK. I see we definitely have some sceptics out there. That's fine. Like I said earlier... Do it. Or don't do it. I don't care.

Is it necessary? No. Cars have been running for years with regular air in the tires for years. But in response to the original question in this thread, let me take a moment to expound on some of the pros and cons.

Nitrogen is a dryer, more stable gas that's less prone to changes in pressure due to heat or cold. Nitrogen has long been used in aircraft tires and in the tires of race cars. Proponents of nitrogen argue that since studies show that fewer than 60 percent of drivers rarely if ever check the inflation of their tires, anything that will slow the normal leakage -- estimated by some at 1 to 2 pounds per square inch a month -- is a good safety measure.

Some experts claim that putting nitrogen in your tires will increase your fuel efficiency because properly inflated tires will reduce rolling resistance, which can mean as much as a 3 percent better mileage than a car with under-inflated tires.

These experts also claim that nitrogen will not degrade the interior rubber of the tire or corrode the wheels, since it contains no oxygen or water vapor -- both present in the atmosphere we breathe and pump into our tires.

It is true that air already consists of 78% nitrogen, so going to pure nitrogen only squeezes out a small amount of the oxygen molecules that nitrogen proponents argue are so detrimental.

Is it worth $40? Probably not. Could it be worth $20 if you're promised a life-time supply of free refills? It probably would be worth it, especially if (like me) you don't religiously check the air pressure in your tires. It might actually help your tires to wear more evenly, allowing you to extend the life of your tires. If I can spend $20 to help enhance the life of some very expensive tires, I'm all for it. That's just my 2 cents.