A tread for a pneumatic tire, the tread comprising: a first circumferentially continuous groove; a second circumferentially continuous groove; a central rib interposed between said two circumferentially continuous grooves (20, 24), the central rib extending continuously around a circumference of the tread; a first circumferentially extending shoulder rib disposed laterally outside of the first circumferentially continuous groove; a second circumferentially extending shoulder rib disposed laterally outside of the second circumferentially continuous groove; the central rib having a plurality of circumferentially spaced sipes arranged in a first circumferential row and a second circumferential row; wherein the sipes of the first row extend between the first circumferentially continuous groove and a circumferentially extending center groove; wherein the sipes of the second row extend between the second circumferentially continuous groove and the circumferentially extending center groove; and wherein, as the tread wears and a radially outermost portion or surface of the tread moves radially inward, the circumferentially extending center groove changes its width or opening between a minimum circumferentially extending axial width or opening and a maximum circumferentially extending axial width or opening; characterized in that the width of the sipes changes continuously or in at least two steps towards the circumferentially extending center groove from a first, axially outer sipe width with respect to the circumferentially extending center groove to a second, axially inner sipe width with respect to the circumferentially extending center groove, wherein the first sipe width is different from the second sipe width. a first circumferentially continuous groove; a second circumferentially continuous groove; a central rib interposed between said two circumferentially continuous grooves (20, 24), the central rib extending continuously around a circumference of the tread; a first circumferentially extending shoulder rib disposed laterally outside of the first circumferentially continuous groove; a second circumferentially extending shoulder rib disposed laterally outside of the second circumferentially continuous groove; the central rib having a plurality of circumferentially spaced sipes arranged in a first circumferential row and a second circumferential row; wherein the sipes of the first row extend between the first circumferentially continuous groove and a circumferentially extending center groove; wherein the sipes of the second row extend between the second circumferentially continuous groove and the circumferentially extending center groove; and wherein, as the tread wears and a radially outermost portion or surface of the tread moves radially inward, the circumferentially extending center groove changes its width or opening between a minimum circumferentially extending axial width or opening and a maximum circumferentially extending axial width or opening; characterized in that the width of the sipes changes continuously or in at least two steps towards the circumferentially extending center groove from a first, axially outer sipe width with respect to the circumferentially extending center groove to a second, axially inner sipe width with respect to the circumferentially extending center groove, wherein the first sipe width is different from the second sipe width.