One other thing that hasn't been talked about here much, is heat build up, under different riding conditions. If you have inflated to the max PSI on the sidewall and you are riding high speed, as on an expressway, for extended times, your tire pressure can increase as much as 4 to 5 PSI, using regular air.The tire manufacturer has factored this in when deciding what the max psi should be on the sidewall. As stated before, an under inflated tire will heat up more than a properly inflated tire.. Keeping the vehicle manufacturer's recommended PSI in your tires is all important.