Originally Posted by
tandemnh
I'm thinking the 12-30 is the trick. I'm going to give this a try first. Adds the extra cog just 3T up without having to go with a new derailleur. I'll get a new chain too.
That's probably the best bet. You may need to adjust the "B" screw so that the rear derailleur's upper pulley can clear the chain. Or it may be necessary to replace the B screw with a longer one or insert the one you have backwards. A hardware store should be able to help you with a longer compatible screw if needed.
Be sure to size the new chain for the big chainring-big cog combination just in case you accidentally cross-chain to that combination. This is to avoid damage.