  We compute the Post-Newtonian parameter (PPN),$\gamma$, for scalar-tensor
gravity theory when the action functional for the scalar field is a
non-standard one, namely the Dirac-Born-Infeld (DBI) type action, used in the
literature for a tachyon field. We investigate two different cases (Linear and
conformal couplings) when the scalar field is non-minimally coupled to gravity
via the scalar curvature. We find that the PPN parameter $\gamma$, which
measures the amount of space curvature per unit rest mass, becomes a function
of the effective mass of the scalar field. Using this PPN parameter, we
calculate the time delay $\Delta\tau$ for the signal to travel the round trip
distance bertween a ground based antenna and a reflector placed in a spacecraft
which is produced due to the grvitational field of Sun. We use this
$\Delta\tau$ to compare our result with that obtained by the Cassini mission
and derive the constraints on the model parameters.
