On the group velocity of oscillating neutrino states and the equal   velocity assumption
  In a two flavour world, the usual equal $p$, equal $E$, or equal $v$ assumptions used to derive the neutrino oscillation length can be replaced by a specific definition of the velocity of the oscillating state which, unlike those assumptions, is compatible with exact production kinematics. This definition is further vindicated by the analysis of $\pi_{\mu 2}$ decay at rest. 
J.-M. Levy