Pneumatic radial tires for heavy vehicles

A pneumatic radial tire comprising:

• an outer covering including a tread portion extending along the outer circumference of the tire, a respective sidewall portion extending from each side of said tread portion, and a respective bead portion extending radially inwardly from each said sidewall portion;
• and
• a carcass comprising carcass plies and belt cord layers disposed beneath said outer covering and reinforcing the same, said carcass plies including carcass ply cords extending radially in the tire, and said belt cord layers interposed between said tread portion and said carcass plies,
• each of said carcass ply cords comprising a twisted composite filament having a core-sheath structure in which a core is surrounded by a sheath,
• said core comprising polyester, and said sheath comprising polyamide and accounting for 40-60 wt % of the entire said core-sheath structure,
• each of said carcass ply cords that comprises the twisted composite filament having a coefficient of twist K defined by ##EQU2## wherein T is the number of twists per 10 cm of cord length, D is the denier of the cord, and
• p is the specific gravity of the cord and the coefficient of twist K being within the range of
• 1. 22<K<2.24, and
• each of said carcass ply cords having an initial modulus Md (g/d) based on the load required to elongate the cord 2% at room temperature satisfying the condition
• Md>45.0.