Having an OBD2 reader is invaluable when driving cross country for the immediate and ongoing fuel economy outputs.
I've been using an ELM327 compatible OBD2 reader with wifi, and connecting it up to my iPhone or iPad with Dash Command.
You can find similar OBD2 readers online for under $30, and the Dash Command app was $50 when we bought it. I like not having extra cables to mess with, and the interface on the app is pretty rich.
Both have paid for themselves in my being able to self diagnose and clear codes for state emissions testing.