47/52 is used by some mfg's (Wilderness Trail Bikes for example)instead of the more familair inch dimensions. It denotes the ratio of tread width to wall height I think. 47/52 is probably closest in size to a 26" x 2.1" tire.
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2003 Iceman Challenge - 2:34:55 - 897 / 2,000*
2002 Iceman Challenge - 2:39:23 - 1093 / 2,186
2000 Iceman Challenge - 2:49:18 - 1516 / 2,153
*estimated