TOPSHAM – Many drivers 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.

But it is 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 tire’s life.

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

Bourque got his tires – filled with nitrogen – in Waterville at the 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.

At Tire Warehouse on Outer Hammond Street in Bangor, staff have been filling tires with nitrogen for close to three years, according to Gary Wilson, the store manager.

“A high percentage of people in this area are getting involved in nitrogen,” Wilson said Friday. “Customer awareness of this has really increased, and the beacon for that has been the price of fuel. We haven’t advertised nitrogen, but once people get it, they tell their neighbors and friends.”

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, vice president of marketing at Tire Warehouse in Keene, N.H.

Nitrogen molecules are bigger than oxygen molecules, so nitrogen seeps out more slowly than from tires that are filled with air; nitrogen resists heat buildup better than air, which contains moisture; and nitrogen reduces oxidation, which can damage the tire from the inside out.

Those advantages are greater 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.

“A perfect tire on average loses a pound of air per month,” Wilson said. “Nitrogen eliminates that because it disperses 20 to 30 times slower than regular air. Plus, nitrogen is inert, so it isn’t affected by temperature.”

When seasons change and temperatures fluctuate dramatically, especially in September and October, a buildup of moisture can form inside tires filled with air, making them more susceptible to deflation, Wilson said.

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,” said spokesman Douglas Love in Yonkers, N.Y.

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

At the Bangor store, many customers who have taken advantage of nitrogen tires have been surprised with the outcome, according to Wilson.

A single mother from the Bangor area, who makes frequent trips to Massachusetts with her daughter, came in to have her tire pressure checked earlier in the week and told Wilson she was amazed that her tires hadn’t lost any air, Wilson said.

One customer reported getting 20 extra miles for every tank of gas with nitrogen tires, according to Wilson.

The difference is noticed more by those who drive compact economy cars compared to trucks and larger vehicles, Wilson pointed out.

“The nuts and bolts of nitrogen is that it extends the wear of your tires, and can help extend your mileage,” he said. “Nitrogen will be there when air won’t.”

Marty Mailhot, manager of the Tire Warehouse in Topsham, said the idea is catching on with consumers, who’re purchasing nitrogen for tires for cars, 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.