Join Date: May 2004
Location: Wilmington, DE
Bikes: 1984 Trek 660, 2003 Specialized Hardrock, 2004 LOOK KG386i (RIP), 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2014 Islabikes CNOC 14 (son's)
Mentioned: 4 Post(s)
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Quoted: 363 Post(s)
For rear deraillers, don't pay any attention to the "speed" designation on it. Rear deraillers have no internal indexing (for that matter, neither do front deraillers but the geometry is different between double and triple deraillers, as well as for different chainring sizes, but I digress). Indexing is done by the shifters so as long as the rear derailler can move along the full width of the cassette, it'll work.
Basically, the only things you need to pay attention to are making sure the derailler pulls from the right direction (look for "top" or "low" normal depending on your setup, check Shimano's website for more info), that it will work with your largest cog (most MTB deraillers will handle up to a 34 tooth cog but many road deraillers are only spec'd up to a 27), and that it has enough chain wrap capacity (if you have a triple crankset, you'll need the long cage version of whatever derailler you go with).
Someone else will let you know if I missed something or made a mistake.