The speedo in my grey CMP gathers speed data by a magnet on the tailshaft, apparently. This has proven to vary wildly, and I do not trust it. Adjustment may make it better, but I still don't trust it.
I went searching for alternatives. Non-standard alternatives. Came across this gauge supplier
http://www.speedhut.com They sell a wide range of instruments, and prices not extortionary. Of interest to me, is their selection of GPS driven speedos, which do not require ANY speed calibration data and arithmetic to obtain extremely close speed tolerences. Naturally, if in an underground carpark or tunnel, your speedo goes dead until you resurface again.
The thing I found REALLY cool with this supplier, is that you can custom make instruments to your own design. I had a little play, and came up with this design with an 'aged' invasion star logo. There are sooooo many areas which can be customised, including dial colors, number font, speed marker lines, speedo needle, bezel color, etc.......
