The key to your car's performance is not limited to it's engine. Tires play a big role in determining how many miles per gallon you get, but many drivers don't check their tire pressure enough.

That's where nitrogen comes in. Some say inflating your tires with nitrogen can help keep your tire pressure from dropping.

Some retailers like Costco are offering customers free nitrogen when they buy new tires.

"I believe in it. It does leak it out less," said Brian Brash, with the Get Nitrogen Institute, a trade group that says industry-sponsored trials indicate truck tires last up to 80 percent longer when inflated with nitrogen.

Brash says a household could save up to $300 a year for every vehicle with tires filled with nitrogen.

So Consumer Reports decided to put that claim to the test. They compared 31 identical sets of tires and rims over the past year. Half were filled with regular compressed air, which already naturally contains about 80 percent nitrogen.

The other half were filled with nearly 100 percent nitrogen.

"From the testing that we've done, we've seen that it doesn't do any harm but it's probably not worth paying for," said David Champion, Senior Director of Consumer Reports..

After a year of sitting outside on racks, the tires with nitrogen lost an average of about 1.3 pounds less than tires filled with regular compressed air.

Champion said that the claim that you don't have to check your tire pressure as often if you fill them with nitrogen is not true.

"Even with nitrogen you still need to check the pressure inside the tires," he said.

The Get Nitrogen trade group says the Consumer Reports study did not take into account variables seen when tires are actually used on the road, and that Americans could save billions in gasoline by using nitrogen-filled tires.

No so, says Consumer Reports.

"If you check your tires regularly, keep them to the vehicle manufacturer's recommended tire pressure, you'll get the best fuel economy," said Champion. "The tires will wear the longest, and you will have a safe vehicle to drive in."

More Information:

ConsumerReports: Nitrogen in Tires