To obtain a compound which can give a vulcanizate lowered in a low-strain modulus without lowering hardness and high-strain modulus to a similar extent by producing a base tread composition and adding a performance improver system containing a specified carbon black, a specified silica and specified kaolin clay and a silane coupling agent to the base tread composition.

One hundred pts.wt. based tread composition (e.g. a composition containing at least either SBR or a natural rubber) is produced. Next, a performance improver system containing 0 to about 60 pts.wt. treat grade carbon black having a high grade structure, about 20 to 50 pts.wt. silica having a high grade structure and a large surface area, and about 5 to about 20 pts.wt. kaolin clay and about 6 to about 12 wt.%, based on the silica and the kaolin clay, silane coupling agent are added to the base tread composition. This means that the dynamic modulus can be lowered without lowering the hardness by replacing a part of the silica by kaolin clay.