So I was looking around for a cheap way to know my instantaneous MPG without buying an expensive scangauge. I ran into an app for Android phones called Torque which lead me to OBDII bluetooth dongles. You can get cheap ones from hong kong for ~$30 or go all out and get OBDKEY from UK for $150. Either way the app takes the data from the dongle and shows it to you on screen. MPG, Tach, G-forces, boost, error codes etc... I'm gonna get one it will take like 2 weeks to arrive but, I'll let everyone know how it goes! The Dongles come with software for PCs but I doubt you'd want a computer in the car with you. A phone sounds more reasonable. As for iPhone users, Android is better

but, I'm sure there is an app similar to Torque on the appstore which can do the same. WinMo phones the same all others, SOL. Enjoy!
Ebay Find (Dongles)
Android software developer's website (Torque)