A passenger-vehicle pneumatic radial tire comprising: a carcass consisting of one or more carcass plies of radially arranged cords toroidally extending between a pair of bead portions; a tread rubber disposed on a tire radial outer side of the carcass, the tread rubber forming a tread surface; and a buttress rubber positioned on a tire widthwise outer side of the tread rubber, the buttress rubber forming a buttress portion, characterised in that: when the tire is assembled to a rim and filled with an internal pressure of 250 kPa or more, an SW/OD ratio of a sectional width SW to an outer diameter OD (mm) of the tire is 0.26 or less if the tire has the sectional width SW of less than 165 (mm), whereas the sectional width SW and the outer diameter OD (mm) of the tire satisfy a relation of 2.135 × SW + 282.3 ≤ OD if the tire has the sectional width SW of 165 (mm) or more, a dynamic storage modulus E' at 30 °C of the tread rubber is 6.0 MPa to 12.0 MPa, and a loss tangent tanδ at 60 °C of the tread rubber is 0.05 to 0.15, a dynamic storage modulus E' at 30 °C of the buttress rubber is 1/2 or less of a dynamic storage modulus E' of the tread rubber, and a loss tangent tanδ at 60 °C of the buttress rubber is 0.1 or less; the tread rubber is disposed in the tread portion located between a pair of virtual lines Le each extending in the tire radial direction and through the tread ground contact end E; and the buttress rubber is disposed in the buttress portion located between the virtual line Le extending in the tire radial direction and passing through the tread ground contact end E, and a virtual line Lh extending in the tire widthwise direction and passing through a tire surface position which is distanced by a half of a peripheral length measured from the tread ground contact end E to another tire surface position which corresponds to the sectional width SW of the tire. a carcass consisting of one or more carcass plies of radially arranged cords toroidally extending between a pair of bead portions; a tread rubber disposed on a tire radial outer side of the carcass, the tread rubber forming a tread surface; and a buttress rubber positioned on a tire widthwise outer side of the tread rubber, the buttress rubber forming a buttress portion, characterised in that: when the tire is assembled to a rim and filled with an internal pressure of 250 kPa or more, an SW/OD ratio of a sectional width SW to an outer diameter OD (mm) of the tire is 0.26 or less if the tire has the sectional width SW of less than 165 (mm), whereas the sectional width SW and the outer diameter OD (mm) of the tire satisfy a relation of 2.135 × SW + 282.3 ≤ OD if the tire has the sectional width SW of 165 (mm) or more, a dynamic storage modulus E' at 30 °C of the tread rubber is 6.0 MPa to 12.0 MPa, and a loss tangent tanδ at 60 °C of the tread rubber is 0.05 to 0.15, a dynamic storage modulus E' at 30 °C of the buttress rubber is 1/2 or less of a dynamic storage modulus E' of the tread rubber, and a loss tangent tanδ at 60 °C of the buttress rubber is 0.1 or less; the tread rubber is disposed in the tread portion located between a pair of virtual lines Le each extending in the tire radial direction and through the tread ground contact end E; and the buttress rubber is disposed in the buttress portion located between the virtual line Le extending in the tire radial direction and passing through the tread ground contact end E, and a virtual line Lh extending in the tire widthwise direction and passing through a tire surface position which is distanced by a half of a peripheral length measured from the tread ground contact end E to another tire surface position which corresponds to the sectional width SW of the tire. when the tire is assembled to a rim and filled with an internal pressure of 250 kPa or more, an SW/OD ratio of a sectional width SW to an outer diameter OD (mm) of the tire is 0.26 or less if the tire has the sectional width SW of less than 165 (mm), whereas the sectional width SW and the outer diameter OD (mm) of the tire satisfy a relation of 2.135 × SW + 282.3 ≤ OD if the tire has the sectional width SW of 165 (mm) or more, a dynamic storage modulus E' at 30 °C of the tread rubber is 6.0 MPa to 12.0 MPa, and a loss tangent tanδ at 60 °C of the tread rubber is 0.05 to 0.15, a dynamic storage modulus E' at 30 °C of the buttress rubber is 1/2 or less of a dynamic storage modulus E' of the tread rubber, and a loss tangent tanδ at 60 °C of the buttress rubber is 0.1 or less; the tread rubber is disposed in the tread portion located between a pair of virtual lines Le each extending in the tire radial direction and through the tread ground contact end E; and the buttress rubber is disposed in the buttress portion located between the virtual line Le extending in the tire radial direction and passing through the tread ground contact end E, and a virtual line Lh extending in the tire widthwise direction and passing through a tire surface position which is distanced by a half of a peripheral length measured from the tread ground contact end E to another tire surface position which corresponds to the sectional width SW of the tire.