TOPSHAM, Maine — Many motorists seeking to improve their mileage as gas prices soar this summer are examining everything — right down to the air in their tires. And for a growing number, plain old air isn't good enough.

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.

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.

And now it's finding its way into the mainstream at a growing number of tire dealers — 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.

Nitrogen molecules are bigger than oxygen molecules, so nitrogen seeps out more slowly from tires than air; nitrogen resists heat buildup better than air, which contains moisture; and nitrogen reduces oxidation, which can damage the tire from the inside out, proponents say. Nitrogen is relatively inert, so there are no safety or environmental issues.

Those advantages are important in vehicles equipped with tire pressure monitoring systems, which are sensitive to changes in tire pressure, McGrath said.

With or without nitrogen, proper inflation is the key to improving gas mileage. Motorists can improve gas mileage by 3.3 percent simply by keeping their tires properly inflated, according to the U.S. Department of Energy.

In the real world, though, only 1 in 5 motorists check tire pressure regularly, according to the Rubber Manufacturers Association. Nitrogen, therefore, could have an advantage for those who don't check their tire pressure regularly.

The National Highway Traffic Safety Administration has no opinion on nitrogen, but it does encourage motorists to keep their tires properly inflated, both for safety and to boost gas mileage, said spokesman Rae Tyson. Severely underinflated tires are dangerous, especially for sport utility vehicles and light trucks, Tyson noted.

Tire experts at Consumers Union, publisher of Consumer Reports magazine, neither endorse nor object to the use of nitrogen in tires.

"Nitrogen is certainly safe to use in tires, and theoretically it does offer some benefits," spokesman Douglas Love said from Yonkers, N.Y.

For Bourque, his tire pressure remains constant — 40 pounds for his fully loaded truck — even on hot days when tire pressure normally fluctuates.

His gas mileage was about 19 mpg when he purchased his five-cylinder 2005 Chevrolet Colorado. Now, with the engine broken in and new tires filled with nitrogen, he gets 20.5 to 22 mpg depending on whether he runs the air conditioner, he said.

For tire dealers, the nitrogen generator and associated equipment typically runs between $3,000 and $12,000, Ulrich said.

Marty Mailhot, manager of the Tire Warehouse in Topsham, said the idea is catching on with consumers, who are purchasing nitrogen for tires for cars, trucks, motor homes and lawn tractors. He has even tried it on footballs and inflatable tubes pulled behind boats.

He has a retort for those who pooh-pooh the notion of paying for nitrogen when there's plenty of free air for the taking.

"I say, 'Why are you drinking that bottled water when there's a pond out back?' " he said.