  We have used a suite of simplified spectrophotometric spiral galaxy evolution
models to argue that there are substantial variations in stellar mass-to-light
ratios (M/Ls) within and among galaxies, amounting to factors of between 3 and
7 in the optical, and factors of 2 in the near-infrared. Under the assumption
of a universal spiral galaxy IMF, our models show a strong and robust
correlation between stellar M/L and the optical colors of the integrated
stellar populations. Using observed maximum disk M/Ls we conclude that a
Salpeter Initial Mass Function (IMF) has too many low-mass stars per unit
luminosity, but that an IMF similar to the Salpeter IMF at the high-mass end
with less low-mass stars (giving stellar M/Ls 30% or more lower than the
Salpeter value) is consistent with the maximum disk constraints. We apply the
model trends in stellar M/L with color to the Tully-Fisher (TF) relation. We
find that the stellar mass TF relation is relatively steep and has modest
scatter, and is independent of the passband and color used to derive the
stellar masses. The difference in slope between the optical (especially blue)
and near-infrared TF relations is due to the combined effects of dust
attenuation and stellar M/L variations with galaxy mass. We find that the
baryonic TF relation has a slope of 3.5 (with random and systematic errors of
+/- 0.2 each). Since we have normalized the stellar M/L to be as high as can
possibly be allowed by maximum disk constraints, the slope of the baryonic TF
relation will be somewhat shallower than 3.5 if all disks are substantially
sub-maximal. [Abridged]
