Hi Brian
I am thinking of going the same route real soon ..... plain gut G & D (probably Gamut) and Garbo A & E. I was also curious about gauge of Garbos so I checked the string gauge sticky here at TalkBass:
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The Garbo gauges are listed there .... I assume they are correct and are for the "regulars" (Velvet also has a "light" gauge Garbo ..... I don't know what those gauges would be).
Page down to Velvet Garbo and then to the right to see the "gauge" in thousandths of inch:
G 0.092 (2.3mm) / D 0.093 (2.4 mm) / A 0.097 (2.5 mm) / E 0.111 (2.8 mm)
As far as converting Damian's Pirastro / Pirazzi gauge to millimeters to thousandths of inches, you can use Damian's convertor here:
http://www.damianstrings.com/calc.htm
or
from the Gamut website:
"Gauging systems ... In the Gamut catalog the plain gut strings are gauged by millimeter diameter .... There are at least three popular gauging systems for measuring strings. Strings are also measured in thousandths of an inch, or "thou", and wound strings are often measured in "pirazzi-meters" . You can switch easily between each system by the following methods.
To change from millimeters to pirazzi-meters divide the millimeter number by .05.
To change from pirazzi -meters to millimeters multiply the pirazzi-meter number by .05.
To change from millimeters to thousandths of an inch divide the millimeter number by 25.4.
To change from thousandths of an inch to millimeters multiply the thousandths number by 25.4."
Now ....if you want those measurements in leagues or miles .... I can't help you there

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P.S. Listen up to what superman has to say .... that feller knows what he's talkin' about

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