Nitrogen filled tires will not give you better mpg. The nitrogen molecule is larger than an oxygen molecule so nitrogen will leak less out of a tire. This keeps proper inflation for a longer period of time so you continue to get better mpg. The same thing can be accomplished by checking your oxygen filled tires more often to keep proper tire inflation. At $6 a tire for nitrogen it is not worth it. Just check your oxygen filled tires more often to continue to get better mpg. No rocket science there.
