An increasing number of local auto-repair shops and car dealerships are charging customers $20 to $50 to fill their tires with nitrogen.

Their pitch is that pure nitrogen maintains better tire pressure, which gives you better gas mileage, longer wear and a smoother and safer ride.

Carfix, a shop in Garner, charges $20 to fill four tires and a spare. Refills are free. The shop has offered nitrogen for two years and now at least 10 percent of customers request it, said the general manager, Mike Allen.

"It's not something we really push hard," Allen said. "We do it as a convenience to customers. Some people want the best of the best."

Capital Chrysler Jeep Dodge in Garner initially charges $49.95 to fill four tires. The price includes other services, such as 24-hour emergency towing. The dealership also refills the tires for free.

The idea of riding on nitrogen isn't new. It's been done for years in race cars, commercial airplanes and long-distance trucks.

More recently, a number of car dealerships across the country are using it in new car tires -- basically, a luxury "extra" to impress buyers.

The thinking is that nitrogen, which makes up 78 percent of the atmosphere (21 percent is oxygen and 1 percent is other stuff), has larger molecules, which prevents it from seeping out of the tire as quickly as air and thereby maintains stable pressure.

Tires filled with regular air tend to fluctuate in pressure level -- increasing during hot summer months and decreasing during the cold season.

Tire pressure is vital.

The National Highway Traffic Safety Administration says that maintaining the proper pressure can improve gas mileage by almost 3 percent.

The government also estimates that every day, underinflated tires cost the nation about 2 million gallons of gas. Low tire pressure can cause wheels to lose traction, which means the car works harder and burns more fuel.

But is filling your tires with nitrogen really a good solution?

"This is still a heavily debated issue,' said Steve Phillips, AAA Carolinas' traffic safety manager. Some Goodyear and Michelin experts have reported that tires lose pressure from the areas around the valves and rims whether they are filled with air or nitrogen, Phillips said.

"So there is no guarantee that you are going to get better pressure," he said.

Phillips worries about what he calls the "fill-it-and-forget-it mentality."

"Our fear is that people will put nitrogen in their tires and never check them again," he said. "They still have to check for tread [wear] and punctures."

According to a survey by Uniroyal Tire, nearly 50 percent of Americans say they check their tires once a month.

But another recent government study indicated that at least 30 percent of vans, cars and trucks have at least one underinflated tire, Phillips said.

AAA is not against nitrogen, Phillips said. But he cautions that you can achieve the same results with regular air and not have to pay.

Phillips recommends that drivers check tire pressure every month. One trick: Use a pressure gauge to check one tire each time you fill up on gas.

Clearly, for those who check tire pressure regularly, there is no real need to pay for nitrogen.

To me, $50 for nitrogen in tires is a bit like paying $2 for bottled water. One company that sells a nitrogen dispenser to auto dealers promotes the machine on its Web site (www.whynitrofill.com) as "an incredible new profit center for your business."

With pump prices getting ready to climb (oil is nearing $90 a barrel), I'm going to need more proof of the cost benefits before converting to "N."