Engine Monitor for $12

Just stumbled on these things ($12). You plug it into your OBDII port in your car and it wirelessly sends data via Bluetooth to your Android phone. The phone program is free...called Torque. You can set up various gauges in analogue or digital.
Link to Amazon

This is great for me because I have older vehicles that don't have a gauge for mpg. 

It will read fault codes and reset them. 

Not that I'm interested, but it can track your 0-60 or 1/4 mile time and all kinds of other stuff. And it's easily moved from one vehicle to another. 

You can set up 7 different pages of user selected data. 

Here's a screenshot of readings for my '05 ford van in the driveway -- 5 minutes after I took it out of the box.

And another pic when I hooked it up to my Ford Focus after I got home from a quick trip.

It will help me get the most out of a gallon of gas when towing my boat.