I have always thought that cadence on it's own doesn't tell you much other than how fast your legs are turning. Shouldn't the discussion also include the gear you are in. You could be pedalling like the clappers in lowest gear but only doing say 15 mph or pedalling half that rate in a high gear and doing 25 mph. I think I must be cycling inefficiently as I am almost always looking to ride in a higher gear at a slightly lower cadence for any given speed - probably putting unnecessary strain on my knees.

I just did some searching on the net and found this interesting (to me anyway) calculator for gears/cadence/speed. I might do some more research on my riding methods using this tool: Bicycle Bike Gear Ratio Speed and Cadence Calculator