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 from the first circumferentially continuous groove to a circumferentially extending center groove extending about the equatorial plane of the tread; wherein the sipes of the second row extend from the second circumferentially continuous groove to the circumferentially extending center groove; 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; wherein the tire comprises a circumferentially extending first shoulder groove disposed in the first shoulder rib and a circumferentially extending second shoulder groove disposed in the second shoulder rib, wherein, as the tread wears and a radially outermost portion or surface of the tread moves radially inward, the first shoulder groove and the second shoulder groove change their respective 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 radial depth of the first circumferentially continuous groove and the radial depth of the second circumferentially continuous groove is in a range of from 30 to 70 percent of the radial depth of the circumferentially extending center groove. 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 from the first circumferentially continuous groove to a circumferentially extending center groove extending about the equatorial plane of the tread; wherein the sipes of the second row extend from the second circumferentially continuous groove to the circumferentially extending center groove; 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; wherein the tire comprises a circumferentially extending first shoulder groove disposed in the first shoulder rib and a circumferentially extending second shoulder groove disposed in the second shoulder rib, wherein, as the tread wears and a radially outermost portion or surface of the tread moves radially inward, the first shoulder groove and the second shoulder groove change their respective 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 radial depth of the first circumferentially continuous groove and the radial depth of the second circumferentially continuous groove is in a range of from 30 to 70 percent of the radial depth of the circumferentially extending center groove.