Newer cars sport an OBD port where you can access real time data from all your car's sensors. DIYer studio CowFish wanted live access to this data, so they built a system with a Raspberry Pi that reads the data and displays it on a screen built into the dash.
Installation of the screen will vary from car to car, but getting the Pi set up is incredibly easy. You'll just need a handful of parts for the Pi, a few bits of software, and you'll be up and running in no time. Head over to CowFish for the full guide.
OBD-Pi: Raspberry Pi Displaying Car Diagnostics Data On An Aftermarket Head Unit [CowFish Studios via Adafruit]