A tire (1), a mounting rim (2) therefor and a circumferentially unstretchable ring (3) for supporting the tread (10) of the tire when running at low or zero pressure. The tire (1) has a radial ply casing (13) on which the points that are furthest apart axially are radially apart close to seats (12B) of outwardly sloping beads (12). The seats (12B) engage sloping seats (23′, 23″) on the rim (2), which may have at least one mounting well (22), at least one cylindrical portion (21) for receiving the supporting ring (3) and rim flange (24).