Speedo gauge

[furia]

My speedo gauge is shot. I could just reaplace the cable but I have a different thing in mind. Considring my diff in not stock rato and my tires and rims are larger the speedo would not be too acurate.
So what I want to do is put in a GPS based speedo. What I need to know is if there is any ELECRONIC gauge that I can fit in the dashboard. I need a regular speedo with needle off any car that is digital based. A digital speedo can be controlled by a PWM signal from a board such as http://www.arduino.cc/ . The PWM signal is generated using the raw data sent to the board by any gps receiver. No big deal.
An issue would be cruise controll. How does that thing work? How does it get its info and in what way?
I know it sounds kinda wierd but it should be a cool project for the cold winter months. Also having an Arduinio board in a car can do a bunch of other very cool "smart" stuff. You can have it close your windows when you lock the car, calculate MPG and give you remaining KM on the tank, and just about anything.