OBD or On Board Diagnostics and OBD-II is a standard for communicating and reading data from a car, its standard across most modern cars and the likelihood is you have a connector in your car which you can read all sorts of data about the car such as RPM, Speed, Temperature and a million other things you wouldn't guess a car was monitoring.
Update - I've taken this prototype to the next stage and used the OBD data to overlay mph, rpm, temperature and throttle position over video taken with the camera board.
I got hold of a USB to OBD2 interface cable for about £10. Pick one up from amazon.com or amazon.co.uk.
I then took a fork of salgar's software from his github repository, https://github.com/roflson/pyobd, as the basis for my program. salgar's software is a fork from a project called pyobd, https://github.com/peterh/pyobd, which is a GUI based application for reading OBD-II data.
I used this as the basis for a program which would connect through OBD-II interface, 'ask' the car which sensors it supported and then read the data sensors in a loop every 0.5 second and write them to the screen.
I see this as step one in a project to log data about a car for other uses.