For some of the "guages" provided with an X-Guage or ScanGuage II, there are some settings so that the guage can translate the inputs and show us the appropriate output.
For our 6.0PSD this was the case. We could set the guage to show instantaneous MPG, but untill we had also set the guage to know it was a 6.0 PSD, the output was not correct.
THis is what it sounds like is happening to you.
Have you made any such adjustments to your Scanguage?
Greg in Austin
2008 Ford 6.0PSD EB/E-PH SMB 4X4 Aluminess f/r bumpers (13.5mpg avg, 15mpg hwy) 52k miles [Texas McBeast]
2006 Toyota Prius (48 to 68 mpg) 120k miles [Penelope]
2013 Jeep Wrangler Unlimited Rubicon (15 to 18 mpg) [Johnnie]
2012 Mitsubishi MiEV (no gas required) ($.50/day in electricity) [Evie]