A tensioner for an automotive engine accessory drive belt system, which system includes an endless belt entraining at least two pulleys, one pulley being connected to a crankshaft and one pulley being connected to a motor-generator unit (MGU; 916), the tensioner comprising: a base mountable to the MGU; a ring having an inner diameter larger than the diameter of the MGU pulley, the ring being slidably mounted to the base, the base retaining the ring axially and guiding the ring for rotation about an axis that is substantially coincident with a rotational axis of the MGU pulley; a bushing (26, 28) disposed between the ring and the base for damping the rotational movement of the ring relative to the base; and characterized by a tensioner arm pivotally mounted to the ring for pivotal movement about an arm pivot axis that is offset from the ring rotational axis; a first tensioner pulley rotatably mounted to the tensioner arm, the first tensioner pulley positioned for engaging an outside surface of a first belt span on one side of the MGU pulley; a second tensioner pulley that is rotatably mounted to the ring for rotation about a rotational axis that is fixed relative to the ring and offset from the ring rotational axis, the second tensioner pulley positioned for engaging an outside surface of a second belt span on another side of the MGU pulley; and a torsion spring acting between the ring and the tensioner arm for biasing the tensioner arm towards the first belt span and for biasing the first and second tensioner pulleys (18, 20) to move towards one another, thereby tending to increase the wrap of the belt around the MGU pulley; wherein the ring is rotatable in response to hub loads received by the first and second tensioner pulleys (18, 20) that occur as a result of the first and second tensioner pulley engagements with the first and second belts spans. a base mountable to the MGU; a ring having an inner diameter larger than the diameter of the MGU pulley, the ring being slidably mounted to the base, the base retaining the ring axially and guiding the ring for rotation about an axis that is substantially coincident with a rotational axis of the MGU pulley; a bushing (26, 28) disposed between the ring and the base for damping the rotational movement of the ring relative to the base; and characterized by a tensioner arm pivotally mounted to the ring for pivotal movement about an arm pivot axis that is offset from the ring rotational axis; a first tensioner pulley rotatably mounted to the tensioner arm, the first tensioner pulley positioned for engaging an outside surface of a first belt span on one side of the MGU pulley; a second tensioner pulley that is rotatably mounted to the ring for rotation about a rotational axis that is fixed relative to the ring and offset from the ring rotational axis, the second tensioner pulley positioned for engaging an outside surface of a second belt span on another side of the MGU pulley; and a torsion spring acting between the ring and the tensioner arm for biasing the tensioner arm towards the first belt span and for biasing the first and second tensioner pulleys (18, 20) to move towards one another, thereby tending to increase the wrap of the belt around the MGU pulley; wherein the ring is rotatable in response to hub loads received by the first and second tensioner pulleys (18, 20) that occur as a result of the first and second tensioner pulley engagements with the first and second belts spans.