Distance traveled per revolution of crank = (teeth in chainring)/(teeth in rear sprocket) * 2.* pi * radius of wheel

speed = (cadence in revolutions per hour) * (teeth in chainring)/(teeth in rear sprocket) * 2.* pi * radius of wheel


I guess the catch is that your speed may be off by 10-20% if you don't measure the radius of your wheel accurately with your weight on the bike and proper pressure in your tires.