This pneumatic radial tire for a passenger car is configured so that the relationship between the cross-sectional width (SW) and outer diameter (OD) of the tire is appropriately restricted. Also, this pneumatic radial tire for a passenger car has either only at least a single main groove (2a) extending in the circumferential direction of the tread, or the main groove (2a) and at least a single auxiliary groove (2b) other than the main groove (2a), the auxiliary groove (2b) having a width of 2 mm or less. In addition, this pneumatic radial tire for a passenger car is configured so that the negative ratio of the main groove (2a) is in the range from 12% to 20%, inclusive.