12-21 is ok for moderately hilly for me. I can spin up 4-5% grades with that without difficulty. I have a 13-23 I haven't used yet, and a 12-28 that came with the wheel I'll be happy to slap on should I travel somewhere mountainous. Chainrings are a slightly odd 51-41. Oh, and I have an 11-19 I bought cheap on a whim for REALLY flat country. Check Ebay for NOS cassettes...that's where all mine came from.