  We study a gas of hard rods on a ring, driven by an external thermostat, with
either elastic or inelastic collisions, which exhibits sub-diffusive behavior
$<x^2 > \sim t^{1/2}$. We show the validity of the usual
Fluctuation-Dissipation (FD) relation, i.e. the proportionality between the
response function and the correlation function, when the gas is elastic or
diluted. On the contrary, in strongly inelastic or dense cases, when the tracer
velocity is no more independent of the other degrees of freedom, the Einstein
formula fails and must be replaced by a more general FD relation.
