We have continuously observed muon intensities from 2000 to the present using the GRAPES-3 multi-directional muon telescope in Ooty, South India. The radial diffusion coefficient and mean free path of the propagation of Galactic cosmic rays (GCR) in the heliosphere have been derived from the regression coefficients of cosmic-ray intensity and solar wind velocity variations. In the present study, we used the same dataset for the years 2000-2021, covering about 5000 days and divided into 26 groups by the solar wind speed to examine the relationship of the amplitude and phase of solar diurnal variation of each group with the average solar wind speed on the corresponding group. This analysis shows that (1) there is a positive correlation between the amplitude and phase of the solar diurnal variation and the solar wind speed, and (2) in particular, the slope of the regression line between the amplitude of the solar diurnal variation and the solar wind speed becomes slower on the high-speed side than around 450 km/s.