During the last five years 2-ply tires have replaced 4-ply in a very substantial percentage of all passenger tire production. This move was primarily motivated by the improved performance potential of the 2-ply construction with particular reference to extended high speed driving. This paper reviews the engineering principles involved and includes a performance comparison on controlled laboratory and proving ground tests. It also includes a comprehensive performance history comparing 2-ply versus 4-ply on millions of tires produced during this five year period. The results leave no doubt that the 2-ply tire has proven itself superior to the 4-ply on an overall basis.