Research

http://www.instructables.com/id/Arduino-Skateboard-Speedometer/

-An arduino connected to a LED screen catching the amount of turns the skateboard wheel does swell as showing how fast it is going.

https://www.youtube.com/watch?v=ozoYZB6xTY4

- Turns, revolution and rotation counter. Hacking a calculator and connecting wires to a reed switch, when it is closed by a magnet it adds up on the calculator.