Artorius,
here is the wheel/tire on my Quad.
the ruler in the picture is US inches.
sorry, i was not trying to be difficult,
but my ruler w/ metric on one side is AWOL
at the moment. honest. :-(
but i will include this info:
1 inch = 25.4 mm
1/16 of an inch (the markings on the ruler) = 1.5875mm
which should get you somewhere.
maybe.
hope it helps.