Low car diet
Join Date: Aug 2007
Location: Corvallis, OR, USA
Bikes: 2006 Windsor Dover w/105, 2007 GT Avalanche w/XT, 1995 Trek 820 setup for touring, 201? Yeah single-speed folder, 199? Huffy tandem.
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With the exception of old (pre-9-speed) Dura Ace and new (10-speed) Dyna Sys MTB, all modern Shimano indexed rear derailleurs and shifters use the same cable pull RATIO, between how much cable is pulled by the shifter and how far over the derailleur moves. So, the "speed" of the rear derailleur is not a factor.
You may be able to get away with a MTB GS (short/medium cage) RD if the capacity of your drivetrain doesn't exceed the wrap capacity of the GS RD, which I believe is 33T. A 46/34T x 11-32T drivetrain just barely makes this: [46+32]-[34+11]=33. So, this drivetrain would work, but if you went to a wider cassette (e,g, 11-32) and/or kept the 48T outer chainring, then you'd need an SGS (long cage).