A method for increasing the hardness of silica/rubber mixtures includes blending with the mixture at least one silane and a hardness-increasing amount of at least one member selected from the group consisting of thixotropic fumed silica, precipitated silica and an MQ resin, wherein Q is SiO.sub.4/2, M is R.sup.1R.sup.2R.sup.3SiO.sub.1/2, and R.sup.1,R.sup.2,and R.sup.3 are the same or different functional or non-functional organic groups, carbon black, a thermoplastic resin or a thermosetting resin.