I recently went from the stock 700x35 Bontrager Invert Hardcase to 700x28 Continental Sport Contact tires. The bike seems livelier and a bit quicker but before I made the switch I went to my lbs told them what I had and asked what sizes would fit. Considering my stock wheel set they though that 28's would be the thinnest I could go. If there is a lbs close to you I would suggest you ask them, advice is usually free.