George Bourque of Fairfield is one of those who's driving around on tires filled with pure nitrogen, the same stuff that NASCAR racers use.

A motorist uses a gauge to check air pressure in a tire Saturday, July 29, 2006, in Portland, Maine. Some motorists are inflating their tires with nitrogen instead of air because nitrogen-filled tires maintains pressure longer and may improve gas mileage. [AP]

Bourque, an engineer, said he has seen a 1 to 1.5 mile-per-gallon increase since he began filling his tires with nitrogen, which is touted as maintaining tire pressure longer and resisting heat buildup on hot summer days.

"I analyze everything," he said.

Nitrogen has been used for years in the tires of race cars, large commercial trucks, aircraft and even the space shuttle.

But it is finding its way into the mainstream at a growing number of tire dealers ¡ª including Costco Wholesale Corp.

Nationwide, fewer than 10 percent of tire dealers offer nitrogen, but the number is growing, said Bob Ulrich, editor of Modern Tire Dealer magazine in Akron, Ohio. Most dealers charge $2 to $5 per tire for the nitrogen fill-up, he said. The dealers generally offer free lifetime refills.

Bourque got his tires - filled with nitrogen - in Waterville, Maine at Tire Warehouse, which has 50 tire dealerships across New England. The nitrogen was part of an installation package when Bourque bought a set of tires.

Skeptics will question how much can be gained by filling tires with pure nitrogen when the air we breathe is 78 percent nitrogen.

The differences are subtle, but important, said Steve McGrath, Tire Warehouse's vice president of marketing in Keene, N.H.

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