A composition suitable for forming a sidewall for a tire includes a rubber composition which includes at least one rubber, an ethylene/propylene/diene terpolymer, and a polyalkylene copolymer. The rubber may include one or more natural or synthetic rubbers, such as butadiene rubbers and isoprene rubbers. The ethylene/propylene/diene terpolymer preferably has an ethylene content of at least 60%. The polyalkylene copolymer may be a copolymer of a polyalkylene and an .alpha.-olefin, which is formed in a polymerization reaction involving a single site catalyst. The polyalkylene copolymer preferably has a high degree of crystallinity, being preferably at least about 90% by weight ethylene, with a melting point in the range of 80.degree. C. 120.degree. C. The presence of such a copolymer in the rubber composition markedly increases the resistance to crack growth and formation of cracks in the tire sidewall.