It really depends on what kind of riding you do, and where you are on your chain and sprockets. I wouldn't necessarily run out and buy new chain and sprockets unless they're ready to be replaced. I also have the Ceros and have been running around for the last year or so on stock gearing. The low first gear is still low enough for most of what I do. If you are ready to replace your chain and sprockets I think you would be happy with a 55 tooth sprocket, unless you spend a lot of time on the open road. That'll put you back to just a little deeper than stock. You can put in the actual size of the Ceros in the sprocket calculator found here and play with it a little bit. The OD of the Ceros is found in this thread.