  We consider wave packets of free particles with a general energy-momentum
dispersion relation $E(p)$. The spreading of the wave packet is determined by
the velocity $v = \p_p E$. The position-velocity uncertainty relation $\Delta x
\Delta v \geq {1/2} |< \p_p^2 E >|$ is saturated by minimal uncertainty wave
packets $\Phi(p) = A \exp(- \alpha E(p) + \beta p)$. In addition to the
standard minimal Gaussian wave packets corresponding to the non-relativistic
dispersion relation $E(p) = p^2/2m$, analytic calculations are presented for
the spreading of wave packets with minimal position-velocity uncertainty
product for the lattice dispersion relation $E(p) = - \cos(p a)/m a^2$ as well
as for the relativistic dispersion relation $E(p) = \sqrt{p^2 + m^2}$. The
boost properties of moving relativistic wave packets as well as the propagation
of wave packets in an expanding Universe are also discussed.
