This hollow black vinyl line is soft and bends easily. 

Inside diameter is .031"

This soft line equals the following equivalent 1:1 size at each scale.
1/25 scale = 1 7/8"
1/16 scale = 1.25"
1/12 scale = 1"
1/8 scale  =  9/16"

Depending on your scale, typical uses are coolant, fuel and oil lines.  Also, at larger scales, A/C lines to/from the compressor and fuel supply and return lines to/from the fuel cell or gas tank.

Each package contains 12" of .075" hollow black tubing, by Parts by Parks.