A pulley comprising: a central hub; a peripheral crown; a first intermediate section comprising a plurality of radial ribs that radially extend from the hub, and a second intermediate section comprising a plurality of peripheral ribs that end into the peripheral crown and intermediate circular rib concentric with the center of the pulley and having a radius basically equal to half radius of the pulley, the circular intermediate rib separating the first intermediate section from the second intermediate section, the circular intermediate rib comprising connection points of radial ribs and of peripheral ribs, wherein the connection points of radial ribs are distant from those of the peripheral ribs; the peripheral ribs being suitably inclined mutually and with respect to the radial ribs in such way to intersect at least in one crossing point to form a web structure in the second intermediate section.