With the price of gasoline nowadays--not to mention increased concern for the environment--everyone is looking for ways to improve fuel economy. Capitalizing on this, some tire dealers and a few service stations are touting the benefits of inflating tires with pure nitrogen rather than ordinary compressed air. Nitrogen, they claim, not only helps tires maintain their full pressure longer, thereby improving fuel economy, but also extends tire life. Retailers who have invested in nitrogen inflation equipment charge as much as $5 per tire for the service.

Long used in race car, aircraft and big truck tires, nitrogen permeates rubber three times more slowly than oxygen. Nitrogen-filled tires also contain less water vapor than those filled with compressed air. These two factors allow a nitrogen-filled tire to maintain more consistent pressure longer and under a wider range of operating temperatures. And a properly inflated tire has less rolling resistance than an underinflated one--which, of course, yields the best possible fuel economy.

"The advantages of nitrogen don't matter as much in a passenger car," says Mark Kuykendall, engineering manager at Bridgestone Firestone. For example, the heat generated by race cars going 200 mph changes the tire pressure and consequently the handling characteristics of these finely tuned machines. Your car's tires, however, run under far less extreme conditions.

And, yes, oxygen does permeate rubber. Typically, your tires lose about 1 to 2 psi per month--a bit of which may be due to permeation. But far more of the total is probably due to slow leaks around the bead (where the rubber meets the rim) and the valve--and nitrogen won't help you there.

Proponents also claim that nitrogen extends tire life by reducing internal oxidation. That's important in big truck tires, says Kuykendall, because they operate at 100 psi, run for two or three times the mileage of passenger car tires, and are eventually retreaded. Realistically, the wear of everyday driving affects your tires' life far more than internal oxidation.

The bottom line: Don't expect to get better fuel economy or longer life from a nitrogen-filled tire than you would from one properly filled with ordinary air. And don't expect to check your tire pressure any less frequently, either. "Nitrogen isn't a bad thing," says Kuykendall. "It won't hurt your tires. But it definitely doesn't relieve you of the obligation to check your tire pressure at least once a month."