ok, a 3 to 1 ratio with 700c wheels (48-16 or close)
90rpm is 20ish mph which is the kind of tempo you can bowl along all day
140rpm (which is the maximum that an average joe can either hold on the flat for shortish bursts or live with downhill without getting flicked over the handlebars when riding fixed) gets to around 30mph
note that I’m not being too specific on the numbers, because the odd tooth change doesn’t make that much difference and your cadence goes up and down but you don’t worry too much about it cos you have no choice 🙂
I ride fixed in and out of work every day and year round, I’m no riding god but I find it good to keep me fit and it’s beautifully low maintenance