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New tires are faster not only because they have better rubber compound qualities over used tires, but also because they have a thicker layer of rubber and that helps to provide a cushion to help fill the gaps in the track surface. As we wear off rubber naturally, the layer of cushion decreases and the tire rides more on the ridges in the track rather than on all of the surface area. Tires that are treated, either legally or not, will conform to the track surface irregularities better and provide more traction. You can overdue softening of the tires to the point of causing excess wear over the same period of time.