Typically our tube diameters are less than 2”; maybe 1 – 1 ½”, which increases the percentage difference a bit.  But also in the typical system the biggest chunk of pressure drop is through the radiator, where I think the velocities in the tubes may get up to more like 10 – 15 ft/sec. 

On further that tube velocity is probably too high, it is more likely that it is roughly 5 ft/sec at cruise conditions, but of course depends entirely on rad design. (In case anybody cared J)

Al