9 (Ahn et al. 2012), and K-band magnitudes from the Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006). Distances are estimated from the 21 cm redshifts (see Mould et al 2000). To minimise the scatter we consider only galaxies with inclinations $> 45^{\circ}$. The final sample after cutting away non-detections and poor-quality detections (see Catinella et al. 2013) consists of $\sim 340$ galaxies.

The comparison between model velocities and observed quantities is complicated by some unavoidable caveats, that we address below.

$\bullet$ In the GASS sample the circular velocity is obtained from HI linewidths. The HI linewidth is an integrated quantity, that we compare with the amplitude of the theoretical rotation curve at one radius. However $r=2.2 R_D$ is large enough for the rotation curve to have already peaked; following Catinella et al. (2006) and Verheijen (2001), we know that in massive galaxies (such as those in the GASS sample) the rotation curve rises rapidly and it remains flat for most of the radial range. Therefore, a measurement of the HI linewidth will be largely dominated by the signal coming from gas that would contribute to the flat region of the rotation curve. Being this the case, the 