A power transmission belt having a body with an inside, an outside, and laterally oppositely facing side surfaces. The body has at least one surface that engages a pulley to frictionally transmit forces between the power transmission belt and pulley. The body includes rubber defining at least a part of the at least one surface. The rubber is a composition made up of a blend of at least: a) 100 parts by weight of an ethylene/a-olefin elastomer; b) 10 to 25 parts by weight of a plasticizer having a solubility parameter of from 8.3 to 10.7 (cal/cm 3 ) 1/2 ; and c) 60 to 110 parts by weight of an inorganic filler.