I'm working on this now. I have an app that I wrote for Windows Mobile, I just have to redo the UI using the Streetdeck com model. You enter the odometer, gallons, and price at each fill up. It gives you MPG and fuel cost per mile for each tank and total. You can also enter brand, grade, and station location if you want to track differences between brands/grades.
It also has reminders for maintenance items, but they are hard-coded for my car, so I need to add some config screens for that.
I also want to have it auto-display the "add fillup" screen when you fill up... based on input from the OBDII interface.
Also have another one working (very early stages). Using the OBDII interface, I want to display speed, injector duty cycle, and throttle position in realtime. With speed and duty cycle you could calculate an instantaneous MPG...