You can get a bluetooth enabled ELM327 interface for £7 and commmunicate with it directly as you drive to get vitals. It's very simple, all commands are 'AT' commands. Just google for the 'ELM327 datasheet' from Elm Electronics and you'll get the idea very quickly.
I modify these units to make a special function OBD device, I desolder the bluetooth module and connect right onto the rx and tx pins.... see here http://www.flickr.com/photos/[email protected]/8207513358/
You can get all the live data you want from OBD, like I say yoiu just need the ELM327 datasheet....yoiu can easily use one of the many graphical output methods of the Pi to display that data