Gear Combo Guru
Join Date: May 2007
Bikes: Spec' Tarmac (road), Ridley Fenix (commuter & fast tourer), Salsa Mamasita (MTB), CoMo Speedster (tandem), Surly Big Dummy (cargo), Airnimal (folder)
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If you do decide to go with an 11-30 cassette then you may be able to get away with not replacing your rear derailleur. Officially, the largest rear cog the derailleur will cope with is 27 or 28 teeth, but on many frames it is possible to use a 30 tooth (it depends partly on the exact shape and angle of your derailleur hanger). Screw the b-screw in all the way, or use a longer screw, and the upper pulley may be able to clear the largest cog when the chain is on the small ring in front and large cog at back. Chain wrap is certainly going to be a problem, but only in the small-small gear combinations. If you avoid using the smallest 3 cogs on the back when in the smaller chainring, then the derailleur should be able to wrap enough chain and it will never be slack or rub on itself in the lower part of the run.
A 9- or 10-speed chain will help you to cross-chain more with the 10-speed crankset without having the chain rub on the outer rings, but you could also stick with an 8-speed chain and put up with the rubbing or avoid cross chaining (you already need to avoid it when in the smaller chainring due to the chain wrap capacity issue). However, when switching from a 12-25 to an 11-30 cassette, you'll probably need to lengthen the chain anyway, so if the current chain has some wear already then it is probably worth going with a new one, and so a 9- or 10-speed chain would make the most sense.