Upper row: correlations between gas and SFR surface densities (left) and volume (centre and right) densities for NGC 5055; h_SFR is assumed to be constant and flaring (Eq. (17)) in the central and right panels, respectively. Each point is obtained as an azimuthal average and coloured according to its galactocentric radius. The slope of the VSF law is much shallower than for the surface-based law. This is a consequence of taking the flaring of the gas (and the SFR) into account. Lower row: same as the upper row but for all the galaxies in our sample. Each galaxy has its own colour as shown by the colour bar. The VSF law has considerably less scatter than the surface based version. Each panel shows similiar ranges in x and y. No obvious break in the SF efficiencies is found at low densities after correcting for disc thickness.