From what I read in Wikipedia, the velocity of a Galaxy has two components: one is due to Hubble's law for cosmic expansion, and the other is the peculiar velocity of the galaxy.
Since the peculiar velocity of galaxies can be over 1.ooo km/s in random direction, this causes an error in evaluating their distance using Hubble's law (I am summarizing from Wikipedia).
A more accurate estimate can be made by taking the average velocity of a group of galaxies: the peculiar velocities, assumed to be essentially random, will cancel each other, leaving a much more accurate measurement.
I assume that "group of galaxies" actually means the cosmic structure by that name, rather than just any collection of galaxies that seem to be in "the same neighborhood", though the Wikipedia text does not explicitly reference that, as it usually would. But I will ignore that issue.
The major problem that I see is that the speed of celestial structures with respect to their "surroundings" seems to be in proportion to their size: 30 km/s for Earth, 200 km/s for the Sun, 600 km/s for the Milky Way, and generally up to 1000 km/s and more for galaxies.
So I would expect this to go up again for even larger structures, such as groups or clusters of galaxies.
Hence, while averaging velocities may give some correction in the measurement, the major source of error should come from the group velocity itself, and would not be corrected by that procedure.
This would weaken significantly the Wikipedia assertion that it produces a "much more accurate" measurement.
Am I right, or do I make an error in my reasonning?
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