The ETRTO number from the side of the tires (example 700x23 is an ETRTO 23-622) will tell you what you need. Use this number if you really want to nail down a proper size. Oddly enough, 27 doesn't always equal 27. But a 622 is always a 622 (which some folks call a 28 and others call it a 29er). The ETRTO number will always be in the format xx-xxx.