A large set of limb coronal mass ejections (CMEs) are used to determine the accurate relationship between radial (V rad) and expansion (V exp) speeds of CMEs. It is demonstrated that this relation is exceptionally well described by the function f(w)=1/2(1+cot w), representing a full cone model for the CME with a half-width, w. We also demonstrate that for extremely fast CMEs (V exp>3000 km s−1), it is better to use the approximation V rad≈V LE. This implies that such CMEs expand spherically above the solar surface.