Patent ID: 8445580

Claim:
A pneumatic rubber tire is having a circumferential rubber tread of a cap/base configuration with an outer tread cap rubber layer with a running surface and an underlying tread base rubber layer, wherein the rubber composition of said outer tread cap rubber is comprised of, based on parts by weight per 100 parts by weight of the tread rubber (phr), (A) about 55 to about 85 phr of specialized cis 1,4-polybutadiene rubber, and (B) about 45 to about 15 phr of at least one additional diene-based elastomer including c is 1,4-polyisoprene rubber, wherein said specialized cis 1,4-polybutadiene rubber, has a microstructure comprised of about 96 to about 99 percent cis 1,4-isomeric units, a number average molecular weight (Mn) in a range of from about 150,000 to about 200,000 and a heterogeneity index (Mw/Mn) in a range of from about 1.5/1 to about 2.1/1, (B) about 40 to about 110 phr of reinforcing filler wherein said reinforcing filler is a combination of rubber reinforcing carbon black and synthetic amorphous silica (precipitated silica) comprised of: (1) about 20 to about 60 phr of rubber reinforcing carbon black, and (2) about 10 to about 80 phr of precipitated silica; and (C) coupling agent having a moiety reactive with hydroxyl groups on said precipitated silica and another different moiety interactive with carbon-to-carbon double bonds of said cis 1,4-polybutadiene rubber.