This paper presents a study on stellar mass-to-light ratios (M/L) and the Tully-Fisher (TF) relation in spiral galaxies. Using galaxy evolution models, the authors argue that there are significant variations in M/L within and among galaxies. Their models show a strong correlation between M/L and galaxy color. They compare the colors and maximum-disk M/L values of a sample of galaxies to the model color-M/L relation, finding that a Salpeter IMF is too massive, but an IMF with fewer low mass stars fits the observations well. Applying their color-M/L relation to the TF relation, they find a stellar mass TF-relation that is independent of the originating passband. Adding the HI gas mass, they find that the maximum slope of the baryonic TF-relation is 3.5. The authors use galaxy evolution models to investigate stellar M/L ratios. These models were tuned to fit the observed trends between colors and structural parameters of spiral galaxies. They consider various models with different star formation laws, gas infall and outflow, and formation epochs. All models show large variations in M/L, but a strong correlation between M/L and optical color is found. The slope of the color-M/L relation is robust against different stellar population synthesis models and galaxy evolution models. The main uncertainty is the zero-point, which is determined by the assumed IMF, particularly the relative amount of low mass stars. They use galaxy rotation curves to constrain the color-M/L correlation zero-point. The stellar disk in a galaxy cannot be more massive than allowed by its rotation curve, resulting in a maximum-disk M/L. They show that a Salpeter IMF over-predicts the maximum allowed mass for many galaxies, but a modified Salpeter IMF is consistent with observations. The authors also study the TF relations, finding that the stellar mass TF-relations derived from different passbands are equal within uncertainties. Adding the HI gas mass, they find that the slope of the baryonic TF-relation must be less than 3.5, significantly lower than found by McGaugh et al. (2000). This is due to their use of stellar M/Ls consistent with maximum disk constraints and exclusion of low luminosity dwarfs with poorly determined inclinations and rotation velocities.This paper presents a study on stellar mass-to-light ratios (M/L) and the Tully-Fisher (TF) relation in spiral galaxies. Using galaxy evolution models, the authors argue that there are significant variations in M/L within and among galaxies. Their models show a strong correlation between M/L and galaxy color. They compare the colors and maximum-disk M/L values of a sample of galaxies to the model color-M/L relation, finding that a Salpeter IMF is too massive, but an IMF with fewer low mass stars fits the observations well. Applying their color-M/L relation to the TF relation, they find a stellar mass TF-relation that is independent of the originating passband. Adding the HI gas mass, they find that the maximum slope of the baryonic TF-relation is 3.5. The authors use galaxy evolution models to investigate stellar M/L ratios. These models were tuned to fit the observed trends between colors and structural parameters of spiral galaxies. They consider various models with different star formation laws, gas infall and outflow, and formation epochs. All models show large variations in M/L, but a strong correlation between M/L and optical color is found. The slope of the color-M/L relation is robust against different stellar population synthesis models and galaxy evolution models. The main uncertainty is the zero-point, which is determined by the assumed IMF, particularly the relative amount of low mass stars. They use galaxy rotation curves to constrain the color-M/L correlation zero-point. The stellar disk in a galaxy cannot be more massive than allowed by its rotation curve, resulting in a maximum-disk M/L. They show that a Salpeter IMF over-predicts the maximum allowed mass for many galaxies, but a modified Salpeter IMF is consistent with observations. The authors also study the TF relations, finding that the stellar mass TF-relations derived from different passbands are equal within uncertainties. Adding the HI gas mass, they find that the slope of the baryonic TF-relation must be less than 3.5, significantly lower than found by McGaugh et al. (2000). This is due to their use of stellar M/Ls consistent with maximum disk constraints and exclusion of low luminosity dwarfs with poorly determined inclinations and rotation velocities.