Originally Posted by
Maddox
I know a lot of people go to a 22 tooth cog in the back in hilly areas such as Boston. I don't know about San Fran, but it is a cheap (<$20) thing to try out. You just pop out the old cog and slide the new one on. Any bike shop should be able to do it for you in 2 minutes.
The chain will probably be a bit too short, though. I'd just replace it at the same time.