Just an FYI to you guys looking to work with microcontrollers integrated with gauges and key engine operations/functions. While the Pi is very useful in certain application (I built a GPS logger for data mining on the track a while back), there are huge downfalls to using an OS based computer to manage functions. Linux can be fairly stable under normal usage but I find with Python going it can be a bit unpredictable; coupled with the excessive boot time and the need for a proper shutdown, you can end up having something that from an automotive perspective can leave you stranded if it fails. I worked in startup procedures to collect data as soon as it came on, but if you were relying on the data in real time a crash could be, needless to say devastating. If you just want to use it for data display then by all means. I suggest coding in python and transmitting to an android tablet. Programming android apps in basic is fairly easy and quick to do. You can overlay a bunch of gauges.
As for arduino, stay away from it at all costs. Arduino is a toy, pure and simple. It doesn't have the speed, memory, or inputs/outputs to handle anything automotive related unless you're running a pile of them in tandem. Depending how you transfer data that could pose problems as well given limits of I2C interfacing and the lengths you have to run in a car. I have built an electronic thermostat control with one before and even that was taxing it pretty good. They're great for education but not for practical use. Skip the middle man and just program the ATMega chip an arduino uses on its own in C. They're very powerful chips, the arduino interface puts huge handicaps on them.