Incidentally...
The surface roughness adds to the flow restriction, as per this formula:
f=0.2083 (100 / C) ^ 1.812 * (Q^1.852 / d^4.8655)
Where
f = friction loss per 100 feet in feet of water
C=roughness coefficient (Hazen-Williams factor)
Q=flow rate (gpm)
d=inside diameter of pipe (inches)
Source:
http://www.ppfahome.org/pdf/pvcpipewaterspec.pdf
For PVC piping, a value of 150 is used for C. A steel pipe would have a C value of about 100, and a rusty one about 80.
While on the topic, the optimal tubing size should allow a flow speed of no more than 8 fps (feet per second), otherwise the tubing becomes a significant restriction. A flow speed of 5 fps is the guideline for 1 inch ID and higher.