I haven't done tests over longer distances as my testing was just repeating the tests from the thread over in the general topics forum.
![Image](http://i.imgur.com/5R7IyMu.png?2)
I used the same setup, just replacing the pipes.
As a result I got the following results back that at-least on shorter distances with pumps larger is better.
These results are in no way indicative that smaller pipes by themselves transfer fluids at slower speeds when used without pumps, or that over longer distances between the pumps larger pipes are still faster than small ones, but I think that it does merit a a more detailed investigation to see how things actually work. I just haven't made time to complete further testing.