It should be "relatively" accurate provided your data is correct in the XFI and your fuel pressure is around 3 bar (45psi). The most common problem is the VSS isn't calibrated correctly. To do this properly, I use a GPS and bring up the View/System Configuration/Sensor Calibration/Rate Sensor Calibration Table. I also have the guage dash displayed in the background so I can compare the XFI mph to gps mph while adjusting the table.
Norb's: I'm not suprised your mpg is better at 45 vs 60