[MetroMPG]

Did you get a manual with your ScanGauge?

First make sure your speedo/odo is reading correctly (GPS comparison, or drive a pre-measured route). The manual explains how to calibrate the ScanGauge speed to real world (you enter a % offset if you find an error in your OEM speedo/odo).

There's also a fill-up procedure that will calculate a fuel volume % offset, or you can calculate the difference in fuel consumed and enter the offset % manually. It's fairly normal for the ScanGauge to be off by a few percent at least.

PS - if you don't have your manual, you can download a PDF from the company's site.