Nitro-fill or putting nitrogen in your tires seems to be one of the latest improvements in tire care. I've seen it offered in some Costco and Pep Boys stores. It's supposed to keep your tires inflated longer resulting in better safety and longer tire life. Is anyone using it? How much did you pay? (I've heard between 2 and 5 bucks per tire.) Do you think it was worth it or is it just another rip-off?