Starter-alternator assembly including a poly-V belt comprising a body made of elastomeric material, a plurality of filiform resistant inserts longitudinally embedded in the body, and a coupling portion integrally connected to the body and comprising a plurality of V-shaped ribs set alongside one another and alternating with V-shaped grooves. The belt has a dynamic modulus, with a tensile force pre-set at 600 N at a frequency 15 Hz, that after 10 000 cycles is higher than 110 000 N/rib/branch, and is advantageously made with resistant inserts obtained from at least one first fiber material and one second fiber material.