Joined: Feb 2012
Posts: 19,381
Likes: 5,527
From: Rochester, NY
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
1st choice - replace with the same kind.
2nd choice- replace with the same gear count and "tooth diameter"
There's some wiggle room, sometimes, with pulleys but duplicating origonal spec is always the best choice. What kind of der do you have? Model # is the best way to be specific. Photos are next best. lAndy.