Dynolicious tracks the changes of speed with the iPhone’s built-in accelerometer, a silicon-based sensor that measures the motion of a vehicle. The software constantly measures these changes and converts the information into a good estimate of how fast the car is going. Another program, called G-Tac, does much the same thing as Dynolicious.
http://www.nytimes.com/2008/10/30/automobiles/autospecial2/30phone.html?_r=1&oref=slogin
