COG compatibility
 

I have ultegra 6700 shifters, rear dérailleur, and a 12-25 cog. Is there a COG that has a 30 tooth gear that would work with my current setup?

A bit more info would certainly help. Such as the number of speeds, short or long cage, and chainring combination.

But in general, RDs have a capacity based on two considerations. The largest sprocket they can clear, and the chain take up capacity, expressed as the sum of the differences largest to smallest, front and rear. For example, if you have 39/53 chainrings, and a 12-25 cassette you need 14+13 or 27 teeth of take up capacity.

Those capacities are listed by the maker on their spec's page. However, there's usually some fudge room, so I find I can routinely exceed them by about 10% or so.

So you can work this out for yourself, by checking system capacity and then shopping among the various cassettes to fit the system.

Any 6700 rd will take a 28t cog and the newer ones will take a 30t by design. 6700A? Anyway, it also depends on the derailleur hanger so you will just have to try it. The chances are good however, and a Tiagra 12-30 cassette is only about $25 so it's worth a try. Need a new properly sized chain as well.