I got this free app for my Samsung Galaxy Prevail cell phone called " GPS Speedometer" free demo and Velcro the phone to the handle bars as I ride I now get a perfect digital speedometer, on the button accuracy ,there is even an rpm option for it. I have no idea how tf that works but it does. Here is what it looks like on the machine!!
Here it is Side ways
BEST APP EVER FOR A PHONE "GPS SPEEDOMETER"