  In the standard inflationary scenario based on a minimally coupled scalar
field, canonical or non-canonical, the subluminal propagation of speed of
scalar perturbations ensures the following consistency relation: $r \leq -
8n_{_T}$, where $r$ is the tensor-to-scalar-ratio and $n_{_T}$ is the spectral
index for tensor perturbations. However, recently, it has been demonstrated
that this consistency relation could be violated in Galilean inflation models
even in the absence of superluminal propagation of scalar perturbations. It is
therefore interesting to investigate whether the subluminal propagation of
scalar field perturbations impose any bound on the ratio $r/|n_{_T}|$ in
G-inflation models. In this paper, we derive the consistency relation for a
class of G-inflation models that lead to power law inflation. Within these
class of models, it turns out that one can have $r > - 8n_{_T}$ or $r \leq -
8n_{_T}$ depending on the model parameters. However, the subluminal propagation
of speed of scalar field perturbations, as required by causality, restricts $r
\leq -(32/3)\,n_{_T}$.

Cosmology and Nongalactic Astrophysics;General Relativity and Quantum Cosmology;High Energy Physics - Theory