What is the difference between the 60a Wire, St Wire, and 3c wire?

The hardness (or durometer) of the tire is what those numbers mean. The higher the number (e.g. 60a), the harder and more durable the tire. Lower durometer (e.g. 40a) rubber is super grippy, but wears quickly. The 3C is a triple-compound (hard underneath, 42a center, 40a sides), and the ST is Super Tacky Rubber (40a). Keep in mind that these are all steel bead tires, which is kind of a pain to work with in my opinion [not in mine-RFdeL]. You can read all the marketing hype straight from Maxxis:

http://www.maxxis.com/products/bicycle/product_detail.asp?id=80

You can read-up on durometer here:
http://en.wikipedia.org/wiki/Durometer

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