Predictions of galaxy luminosity from TF and our rewritten FP for disk galaxies. We compare the predicted luminosities (x-axis) to measured luminosities using the galaxies Hubble distance (y-axis) for galaxies from the Pizagno et al. [10] study using their best-fit TF relationship (a) to and that using our reexpression of the Cappellari et al. [29] FP relation and the use of the parameter (b). The scatter is evidently smaller in (b) (0.75 in (a), 0.47 in (b)), showing that including a radial scale, as is done in the FM, adds significant information.