  We study how the recent experimental results constrain the gauge sectors of
U(1) extensions of the standard model using a novel representation of the
parameter space. We determine the bounds on the mixing angle between the
massive gauge bosons, or equivalently, the new gauge coupling as a function of
the mass $M_{Z'}$ of the new neutral gauge boson $Z'$ in the approximate range
$(10^{-2},10^4)\,$GeV/$c^2$. We consider the most stringent bounds obtained
from direct searches for the $Z'$. We also exhibit the allowed parameter space
by comparing the predicted and measured values of the $\rho$ parameter and
those of the mass of the $W$ boson. Finally, we discuss the prospects of $Z'$
searches at future colliders.
