  We study the use of type Ia supernovae (SNe Ia) in the context of
scalar-tensor theories of gravity, taking as a working example induced gravity,
equivalent to Jordan-Brans-Dicke theory. Winking at accurate and precision
cosmology, we test the correction introduced by a time variation of the
Newton's constant, predicted by scalar-tensor theories, on the SNe distance
modulus relation. We find that for induced gravity the coupling parameter is
constrained from $\xi < 0.0095$ (95\% CL) using Pantheon SNe data alone down to
$\xi < 0.00063$ (95\% CL) in combination with {\em Planck} data release DR3 and
a compilation of baryon acoustic oscillations (BAO) measurements from BOSS
DR12. In this minimal case the improvements in terms of constraints on the
cosmological parameters coming from the addition of SNe data to cosmic
microwave background (CMB) and BAO measurements is limited, $\sim7\%$ on the
95\% CL upper bound on $\xi$. Allowing for the value of the gravitational
constant today to depart from the Newton constant, we find that the addition of
SNe further tightens the constraints obtained by CMB and BAO data on the
standard cosmological parameters and by 22\% on the coupling parameter, i.e.,
$\xi < 0.00064$ at 95\% CL. We finally show that in this class of modified
gravity models the use a prior on the absolute magnitude $M_B$ in combination
with the Pantheon SNe sample leads to results which are very consistent with
those obtained by imposing a prior on $H_0$, as happens for other {\em
early-type} models which accommodate a larger value of $H_0$ compared to the
$\Lambda$CDM results.
