In the effort to improve gas mileage and wear on tires, some consumers are filling their tires with nearly pure nitrogen instead of the standard compressed air. While tire makers say their tires are designed for plain air, others say nitrogen provides noticeable benefits. Let's look at the available data and see if there really is a difference.
First of all, maintaining the manufacturer's recommended tire pressure results in the best gas mileage regardless of what's in the tire. Running a tire at 3.5 psi below that recommended pressure results in about a 1% lower fuel efficiency on average. In other words, if 1 tire is 3.5 psi low and your car normally gets 25 mpg, you'd expect only 24.75 mpg.
Nitrogen bleeds through your tires less efficiently than oxygen, meaning that your tires should lose pressure slower with a nitrogen fill than with a standard air fill. Remember, though, that ordinary air is already about 78% nitrogen. The National Highway Traffic Safety Administration conducted a series of test in 2009 which found that indeed, tires filled with nitrogen lost pressure only two-thirds as fast as those filled with normal air. If your tires would normally need to be topped off with air every four months, then with nitrogen-filled tires would need a top-off every six months. However, as the tire aged, the difference between the two dropped off.
Either way, it is maintaining proper tire pressure with periodic checks and aiding either air or nitrogen as needed that results in optimum gas mileage. Tires properly inflated with air get the same mileage as tires properly inflated with nitrogen, but you may be able to go a little longer between checks with nitrogen.
The other claimed benefit of nitrogen is that it results in less tire wear. The NHTSA did, in fact find reason to believe that lowering the oxygen content inside the tire did result in a significantly slower breakdown of the rubber compounds inside the tire, increasing the durability of the tire over time.