On my old fixed, i ran a 42/17 with 700c knobbies. that may seem kinda low, but i lived in san francisco, and the bike was burdened with the full metal basket.
current ss is 40/20 or 40/18, depending on what i think the ride's gonna be like.
the fixed i'm building up is going to be something in the mid 60s, in terms of gear inches.
i like my knees, they just don't like me.