Gravity induced mixing of two miscible fluids in a long tube is studied as a function of the tube tilt angle from vertical $\theta$ and of the density contrast characterized by the Atwood number $At$. At high $At$ values and/or low $\theta$values, the relative concentration of the two fluids follows a macroscopic diffusion law characterized by a diffusion coefficient $D$ increasing strongly with the tube tilting (by a factor of $100$ between $0$ and $70^{o}$). At higher $\theta$ values and/or for low density contrasts, a segregation of the two fluids in the tube section is induced by gravity resulting in a stable counterflow with little mixing at the interface. The Atwood number corresponding to the transition between the diffusive and counterflow regimes increases strongly with the deviation angle from $At = 10^{-4}$ for $\theta = 0^{o}$ to $At =\,5\times 10^{-2}$ for $\theta = 80^{o}$ .