  We compute the propagation speeds for a conformal real relativistic fluid. We
begin from a kinetic equation in the relaxation time approximation, where the
relaxation time is an arbitrary function of the particle energy in the Landau
frame. We propose a parameterization of the one particle distribution function
designed to contain a second order Chapman-Enskog solution as a particular
case. We derive the hydrodynamic equations applying the moments method to this
parameterized one particle distribution function, and solve for the propagation
speeds of linearized scalar, vector and tensor perturbations. For relaxation
times of the form $\tau=\tau_0(-\beta_{\mu}p^{\mu})^{-a}$, with $-\infty< a<2$,
where $\beta_{\mu}=u_{\mu}/T$ is the temperature vector in the Landau frame, we
show that the Anderson-Witting prescription $a=1$ yields the fastest speeds.
