  A simple integral relation is obtained for the distribution functions of jet
speeds and jet-counterjet flux asymmetries of classical double radio sources.
Analytic expressions relating the mean jet speed and the mean jet flux
asymmetry, and their variances are derived. The results are considered in the
light of orientation-based unified schemes, and an analytical equation for the
critical angle separating quasars and radio galaxies is derived. The mean jet
speeds of classical double radio sources are estimated using the asymmetry of
jet-counterjet flux densities taken from the 3CRR and B2 samples. For FRI radio
sources the mean jet speed is ~0.54c (with a standard error SE=0.03c), while
for FRII low-redshift radio galaxies and intermediate redshift quasars the
values found are ~0.4c (SE=0.06c) and >0.6c respectively.
