We have studied the relationship between the solar-wind speed [V] and the coronal magnetic-field properties (a flux expansion factor [f] and photospheric magnetic-field strength [Bs]) at all latitudes using data of interplanetary scintillation and solar magnetic field obtained for 24 years from 1986 to 2009. Using a cross-correlation analyses, we verified that V is inversely proportional to f and found that V tends to increase with Bs if f is the same. As a consequence, we find that V has extremely good linear correlation with Bs/f. However, this linear relation of V and Bs/f cannot be used for predicting the solar-wind velocity without information on the solar-wind mass flux. We discuss why the inverse relation between V and f has been successfully used for solar-wind velocity prediction, even though it does not explicitly include the mass flux and magnetic-field strength, which are important physical parameters for solar-wind acceleration.