  The group of coordinate transformations for 5D noncompact Kaluza-Klein theory
is broader than the 4D group for Einstein's general relativity. Therefore, a 4D
quantity can take on different forms depending on the choice for the 5D
coordinates. We illustrate this by deriving the physical consequences for
several forms of the canonical metric, where the fifth coordinate is altered by
a translation, an inversion and a change from spacelike to timelike. These
cause, respectively, the 4D cosmological 'constant' to become dependent on the
fifth coordinate, the rest mass of a test particle to become measured by its
Compton wavelength, and the dynamics to become wave-mechanical with a small
mass quantum. These consequendes of 5D covariance -- whether viewed as positive
or negative -- help to determine the viability of current attempts to unify
gravity with the interactions of particles.
