in adhesive friction on icy roads also before falling off from the tread surface further since in the ebonite the unsaturated bouble bonds of a rubber are almost completely saturated by sulfur the ebonite powder incorporated in a rubber composition does not form a chemical bonding with a matrix rubber of the composition even by the curing of the composition thus the ebonite powder exposed to the tread surface can fall off therefrom during running after the ebonite powder has fallen off the pores formed thereby on the tread surface serve in the same manner as cells of an expanded rubber to contribute to increase in digging friction and adhesive friction moreover before the ebonite powder appears at the tread surface and fails off therefrom it can be present as an additive in the rubber without softening the tread rubber and adversely affecting the durability even if the ebonite powder is incorporated in a rubber composition in an amount necessary and sufficient to improve the grip characteristics the average particle size of the ebonite powder used in the present invention is not more than 100 u03bcm if the particle size is more than 100 u03bcm the local difference in hardness of the tread surface becomes large to decrease the adhesive friction since the ebonite powder is a hard materials also the pores formed on the tread surface by falling off of the powder become too large to deteriorate the abrasion resistance further incorporation of the ebonite powder having such a large particle size may deteriorate the grip characteristics on general roads and is apt to become a cause of chipping of the tread rubber the ebonite powder having an average particle size.