A three-dimensional lattice-Boltzmann model which yields correct hydrodynamics at the Navier-Stokes level of the Chapman-Enskog expansion requires a minimum of 26 velocities. We present results for a model with one additional velocity, determined by maximizing the equilibrium entropy. For compressible Rayleigh-Benard convection the model is more accurate but considerably less stable, than a previous, approximate 21-speed model.