Averaged 17.3mph over 12 miles
I timed myself over a local circuit I am very familiar with and my average speed came out at 17.3mph. This was with 52/20 sprockets. The cadence works out to be about 83 I think. Since this I have switched to a 50t front chainwheel so the bike will be a bit slower, but this is working out to be very good gearing for general use in my town, I usually don't have to go up any grades steeper than 5%-6%. This is with a coaster-braked bike, and the coaster brake is supposed to have some friction drag compared to other hubs, which I think is true, but it does not seem to be noticeable especially after getting a few hundred miles on the bike and breaking things in a bit.