Wow.
What you need is an MPGuino. Please see this subforum here on EcoModder.
Cost was about $50 when I got mine not quite two years ago. It's very accurate, reads fuel injector pulses and gets distance from the car's speed sensor.
OpenGauge / MPGuino FE computer - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
MPGuino should work on any gasoline powered car with electronic fuel injection and an electronic VSS. I think it will also work on diesels as long as they have the same. Diesels with mechanical injection won't work unless you can somehow generate the electronic signal, I believe.