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Inflating car tires: Air or nitrogen?

As if the tangled task of maintaining your vehicle without draining your bank account isn't tough enough, some of the folks who sell and service the tires on your car have come up with another item for your consideration:

Would you like regular air or nitrogen with your tires?

Nitrogen?

OK, expect to be charged as much as $10 per tire for that choice.

Is it worth it? Let's check the basics.

You might trace this latest kink to the concerns of several years ago about blowouts on SUVs, particularly the Ford Explorer equipped with Firestone tires. What's at issue with nitrogen versus plain old air, which comes from the pump at the gas station, is that 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. Its use in average road cars is new and controversial.

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.

One Web site that lays out the claims for using nitrogen is www.getnitrogen.org.

It says, essentially, 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.

It also claims 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.

But a closer examination of the facts makes some of the claims for nitrogen seem at best anecdotal or illusory for everyday drivers.

First, the air around us is already 78 percent nitrogen, with 21 percent oxygen and 1 percent other gases. So going to pure nitrogen only squeezes out a small amount of the oxygen molecules that nitrogen proponents argue are so detrimental.

Also, the advantage of nitrogen being more stable and less prone to changes in pressure due to heat in the tires seems of little benefit to average drivers. Race teams use it because they can change the handling of the car by adjusting individual tire pressure by as little as a quarter pound. So having a gas that's ultra stable has real benefits when dealing with such small degrees.

Nitrogen proponents say that the nature of the gas means it's less prone to leaking out over time through the pores present in rubber tires. But most air leakage in tires can be traced to poor fit around the rim of the wheel or the valve stem, rather than gas permeating through the rubber.

Claims of nitrogen being more friendly to the rubber and wheels is also questionable, since most tires wear out the tread on the outside long before the inner rubber would go bad from exposure to oxygen. The same factors hold true for wheels, many of which are made from alloys, not straight steel. You're far more likely to damage a wheel from hitting a curb than see a wheel go bad from oxidation.

A good site that takes a contrarian point of view on nitrogen in passenger car tires is www.eng-tips.com, which is run by engineers.

When it comes down to a dollar decision, it's hard to argue that spending as much as $40 for nitrogen in a set of tires is a good fiscal move.

Even if you accept the arguments of proponents, at some point you are going to have to add air to your tires -- not even the most ardent nitrogen pushers will say that your tires will never lose pressure. When that happens, you're most likely to go to the corner gas station, put in a couple of quarters and pump your tire up with regular old air, which will mix with the nitrogen and degrade its benefits.

Save your money and just keep an eye on your tire pressures.

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