Nitrogen as an inflation gas for passenger car and light truck tires use is widely available commercially. Consumers are confronted with a bewildering selection of offerings, and suppliers tout the purity of their nitrogen generation systems and effectiveness of using the gas in place of air. This paper develops models for the initial tire nitrogen purity, the inflation pressure loss rate, and the evolution of the nitrogen gas purity in the tire as a function of the gas used to top off the tire over its life. A series of simulations using the basic model is developed for air and various purities of nitrogen initial inflation with monthly top-off using air or various purities of nitrogen. The initial inflation pressure loss rate is shown as a function of the tire's initial nitrogen purity. This paper proposes the use of the total oxygen passing through the tire over its lifetime as a metric for evaluation of various inflation schemes. This metric is developed for several of the popular available nitrogen inflation purities using both air and nitrogen as a top-off gas.