So how do I do with with an Arduino?  I had no idea other than there is some way to get the information from the OBD port.  Well it seems that you can use one injector lead and do this, there is even a project that spends time working on this.  Presenting the MPGuino!!

You can build this yourself, or you can get a kit for 40$.

Here is their main page

Here is their wiki

Here is a forum post with schematics

Here is how to put it in your car

I will hopefully order one of these soon and get to play with it.