I used n2 when I got the new tires & wheels mounted cost an extra $20 for the set. Was susposed to help improve mpg, keep tire pressure full durring cold months. But really I have actually seen a decline in my mpg from 18-19 to 15-16. Had the n2 since March 2011 first thing changed on the truck then flowmaster exhaust, & the mpgs are slipping down instead of improving. Maybe a different problem but I'm thinking about cutting my losses & going back to air.