Switching Cassettes
 

I have a 12-25 10 speed Ultegra cass. on my road bike, and need another cassette to use for the trainer wheel I recently picked up. If I buy a 12-27, will I need to put on a longer chain, or is the 2 tooth difference so minor that it won't really matter?

Probably not, but it depends on how short your chain is now. It also depends on how many miles the 12-25 has on it. Old chains and new cassettes don't like each other most of the time.

Heck, get something really different. Like 11-23. You might like it, especially with a triple.

i went from a 14-28 to an 11-34 on my seven speed and no problems with the chain