Originally Posted by
dobber
If it is indeed the long cage, you should be able to go up to a 39 tooth, I'd look at something like a 36
No, the cage length has NOTHING to do with how large a rear cog the rear derailleur will accept. That's a function of the upper pulley's geometry and the derailleur hanger's length. The cage length only controls how much "wrap" capacity the derailleur provides.
Shimano road rear derailleurs have a stated maximum rear cog of 27 or 28T depending on the model and completely irrespective of short or long cage. Experience has shown that on some frames they will work with a larger rear cog but, again. it has nothing to do with cage length.