I've got a 48 up front with a flip flop 17T/18T, both are black all city cogs with cutouts for da lightweight. They're good cogs that seem to be machined well and they're pretty damn quiet! I live in a pretty flat area and everywhere I ride is flat but I have a fair bit of stop start traffic so I run the 18T most of the time. 17T would be good for long continuous rides but if your knees are hurting you should probably drop down to an 18T!