well im trying to put a bike speedometer on my 300ex. i have tried once and failed with a wireless one. what kind do i need? and where is the best spot to mount the computer and magnet

 

Wireless bicycle speedometers do not work well on motorized vehicles because the radio interferences caused by the the electrical system of the quad/motorcycle.

Use a wired model, and glue the magnet to a brake disc. Mount the display unit where you can easily view it.

Depending on your quad, Trail Tech may have a custom speedometer/computer "ready to go". Check them at Trail Tech - Innovative Offroad Solutions at Trail Tech Home. The Endurance speedo runs about $80.