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 an inert gas, 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 US$3,000 and US$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.