  We have examined the trend between CO and HI linewidths by compiling the
linewidths of 219 nearby galaxies. The trend is that the CO linewidths of fast
rotating galaxies tend to be larger than the HI linewidths, and that the HI
linewidths of slow rotating galaxies tend to be larger than the CO linewidths,
whereas the intermediately rotating galaxies have almost equal values. We have
explained the trend using the synthetic rotation curve model, which provides
the linewidth - absolute magnitude relations at any radii, and concluded that
the trend has been occurred by the different distributions of CO and HI in a
galaxy and the gradient of the rotation curves. The exact linewidth should be
corrected for the effects of the gradient of the rotation curve and the gas
distribution.
