I have never heard of any calculation for nitrogen to be used to combat a burning tire, it is most obviously useless for that purpose and secondly wheel well fires are rare and the first thing you are going to do if you detect one is drop the gear. An inert gas is needed as it is non reactive with petrol products. The tire needs a method of thermal protection, if a plug melted and compressed air started to discharge it would fuel the fire. No need to think further into the subject.