This morning there was a sidebar on NPR about people filling their car tires with Nitrogen (alla Nascar as mentioned above) I forget the reference but they said that the cost was about 2-5 $ per tire, and that the savings added up to 1 m/ga. They didn't mention how often you would have to refill so the cost benifit is still a question. So I guess the idea is that if you filled your bicycle tires with nitrogen there would be an improvement in efficiency. Maybe I'll stop by the industrial gas supply place along my commute on the way home and give it a try.