We consider the effect of dark matter on the Tully-Fisher relation for spiral galaxies. We show that a significant fraction of the scatter in this relation could stem from a systematic dependence of the ratio of dark to luminous mass on luminosity. The dynamical effect of the dark matter can be accounted for once a mass decomposition procedure, based on photometry and rotation curve data, has been carried out. We show that the contribution to the circular velocity from the luminous disc alone correlates better with luminosity than does the total velocity, which contains an additional contribution from the dark halo. This correlation is closer to a linear law, has less scatter, and is subject to fewer biases than the conventional Tully-Fisher relation. As an illustration, we use two different sets of distance moduli for a sample of local galaxies to calibrate our relation. Applying Teerikorpi's prescription to correct for Malmquist bias, we then derive values of the Hubble constant of H0 = 61 +/- 5 and 66 +/- 6 km s-1 Mpc-1 for our two sets of calibrators respectively.
