Update:

I received the ScanGauge today (that was fast).

Plugged it in and it does everything they claim it can do.

I was really impressed by all of the user adjustable features:

-Gas tank size
-Engine displacement
-Tire Size
-When you fill up, if you used more or less gas than what it calculated for you, you can enter the actual amount used, and it will automatically calculate a correction factor (I haven't used this feature yet)

After setting all of these up correctly, I should technically get very accurate results. However, it will be a few weeks before I can compare the Scangauge to hand calculated values to see the difference.

This thing has lots of features. The best way to see all of them, is to download the pdf manual here .

The only thing that isn't great about this unit is it's looks. It's in a cheap plastic box (like something you would buy from Radio Shack). I don't mind it too much and I will post some pictures of the unit installed in my X soon.