If you run often should at least get 2 seasons on the tires, Biggest killer is sunlight-and not getting them off the ground for long periods of time full of air (off season). Tires on our level of racing usually rot before we wear them out. Ive run as much as 3 seasons on my Goodyears and watch the circumference crack where the sidewall and slick come together. Now I run "index" type racing, and don't worry about the last little bit of ET, as I can compensate. Straight "Heads up " racing where the ET is crucial, Fresh tires can make a difference on ET, but it is very costly
Cant help with the NO2, not legal in our race formats