  We find a new Tully-Fisher-like relation for spiral galaxies holding at
different galactocentric radii. This Radial Tully-Fisher (RTF) relation allows
us to investigate the distribution of matter in the optical regions of spiral
galaxies. This relation, applied to three different samples of rotation curves
of spiral galaxies, directly proves that: 1) the rotation velocity of spirals
is a good measure of their gravitational potential and both the RC's amplitudes
and profiles are well predicted by galaxy luminosity 2) the existence of a dark
component, less concentrated than the luminous one, and 3) a scaling law,
according to which, inside the disk optical size: $ M_{dark}/M_{lum} = 0.5
(L_B/10^{11}L_{B\odot})^{-0.7}$.
