A tyre has a tread portion having a tread ground contacting surface and a main groove having a bottom surface and two groove wall surfaces extending outwardly in a tyre radial direction from the bottom surface. In a lateral cross section perpendicular to a length direction, at least one groove wall surface has an inner groove wall portion connected with the bottom surface and a protruding portion connected with the inner groove wall portion and having a protruding end protruding inwardly in a groove width direction from a groove reference line obtained by extending the inner groove wall portion. The protruding portion has an inclined surface portion extends obliquely between the protruding end and the tread ground contacting surface. When the tread portion in a reference state is loaded with a tyre load of 1.20 times a standard tyre load, the whole inclined surface portion contacts a flat surface.