  A promising method for measuring the cosmological parameter combination
fsigma_8 is to compare observed peculiar velocities with peculiar velocities
predicted from a galaxy density field using perturbation theory. We use N-body
simulations and semi-analytic galaxy formation models to quantify the accuracy
and precision of this method. Specifically, we examine a number of technical
aspects, including the optimal smoothing length applied to the density field,
the use of dark matter halos or galaxies as tracers of the density field, the
effect of noise in the halo mass estimates or in the stellar-to-halo mass
relation, and the effect of finite survey volumes. We find that for a Gaussian
smoothing of 4 Mpc/h, the method has only small systematic biases at the level
of 5%. Cosmic variance affects current measurements at the 5% level due to the
volume of current redshift data sets.
