A rubber composition for tires, the rubber composition comprising: a diene rubber; silica; and a silane coupling agent; wherein the silane coupling agent is a polysiloxane having an average composition formula represented by general formula below; a b c d (R 1) e SiO (4-2a-b-c-d-e)/2 in the formula, A is a bivalent organic group containing a sulfide group; B is a monovalent hydrocarbon group having from 5 to 10 carbons; C is a hydrolyzable group; D is an organic group containing a mercapto group; R 1 is a monovalent hydrocarbon group having from 1 to 4 carbons; and a to e satisfy the relationships 0 ≤ a < 1, 0 ≤ b < 1, 0 < c < 3, 0 < d < 1, 0 ≤ e < 2, and 0 < 2a + b + c + d + e < 4; and at least one of a and b is not 0; wherein a function v of a shear modulus G' of the rubber composition in vulcanized form at 0.28% shear stress at 110°C, a CTAB specific surface area (CTAB) of the silica, a volume fraction (φ) of the silica in the rubber composition, and a stress at 100% elongation (M 100) of the rubber composition satisfies the relationship of equation (i) below, and the function v is a real number not less than 2.5 and not greater than 20; G ′ = K × v in equation (i), G' is the shear modulus (MPa) at 0.28% shear stress at 110°C, K is a coefficient greater than 0 and not greater than 450, and v is a function represented by equation (ii) below and is a real number not less than 2.5 and not greater than 20; v = φ × 1 + 0.0258 × CTAB × M 100 in equation (ii), φ is the volume fraction of the silica in the rubber composition, CTAB is the CTAB specific surface area (m 2 /g) of the silica measured in accordance with JIS K6217-3, and M 100 is the stress (MPa) of the rubber composition at 100% elongation at 23°C measured by performing tensile testing of the rubber composition in vulcanized form_at 500 mm/min at 23°C in accordance with JIS K6251. a diene rubber; silica; and a silane coupling agent; wherein the silane coupling agent is a polysiloxane having an average composition formula represented by general formula below; a b c d (R 1) e SiO (4-2a-b-c-d-e)/2 in the formula, A is a bivalent organic group containing a sulfide group; B is a monovalent hydrocarbon group having from 5 to 10 carbons; C is a hydrolyzable group; D is an organic group containing a mercapto group; R 1 is a monovalent hydrocarbon group having from 1 to 4 carbons; and a to e satisfy the relationships 0 ≤ a < 1, 0 ≤ b < 1, 0 < c < 3, 0 < d < 1, 0 ≤ e < 2, and 0 < 2a + b + c + d + e < 4; and at least one of a and b is not 0; wherein a function v of a shear modulus G' of the rubber composition in vulcanized form at 0.28% shear stress at 110°C, a CTAB specific surface area (CTAB) of the silica, a volume fraction (φ) of the silica in the rubber composition, and a stress at 100% elongation (M 100) of the rubber composition satisfies the relationship of equation (i) below, and the function v is a real number not less than 2.5 and not greater than 20; G ′ = K × v in equation (i), G' is the shear modulus (MPa) at 0.28% shear stress at 110°C, K is a coefficient greater than 0 and not greater than 450, and v is a function represented by equation (ii) below and is a real number not less than 2.5 and not greater than 20; v = φ × 1 + 0.0258 × CTAB × M 100 in equation (ii), φ is the volume fraction of the silica in the rubber composition, CTAB is the CTAB specific surface area (m 2 /g) of the silica measured in accordance with JIS K6217-3, and M 100 is the stress (MPa) of the rubber composition at 100% elongation at 23°C measured by performing tensile testing of the rubber composition in vulcanized form_at 500 mm/min at 23°C in accordance with JIS K6251.