For years, truckers, racecar drivers, cyclists and the U.S. military have been filling tires on their vehicles with dry nitrogen gas. Why? Inflating tires with nitrogen will extend tire life by up to 25 percent and will improve fuel efficiency by up to four percent. Ever wonder why a car you haven´t driven in years has flat tires? Because in a tire filled with compressed air, the oxygen molecules seep through the wall of the tire over time. Nitrogen molecules migrate 3 to 4 times more slowly than oxygen, so tires stay properly inflated longer.In addition, nitrogen provides more consistent tire pressure and reduces or eliminates the need to check tire pressures and top off tires. Nitrogen-filled tires run cooler and lessen rim corrosion and stem clogging.Companies such as Branick Industries, of Fargo, N.D., and the Parker Hannifin Corp. build nitrogen inflation systems used in garages and service centers. Costco is filling new tires with nitrogen for free. You can get filled up with nitrogen at dealerships, though some dealers charge $2 per tire and up to $5 apiece on tires not sold by them.