ECM MPG vs hand figuring?

How much have you found your ECM to be off, compared to hand figures? Reason I ask, my old T600 is doing good by the Scangauge that's hooked up to it. Even if its off a a good Percent. The series 60 11.1 is still doing good. Truck holds 475 gallons of fuel. Is a factory single axle. Direct 10 speed, 2.93 rearend, 295/75R22.5 tires.
Pete

 

Thanks guy's, mine is showing high. Empty last Friday it showed 14.2 MPG. Loaded coming back, but only 16,000 lbs of Pellets on pallets. 12.7 MPG. This was driving 55 almost the whole 4 hours. Not much in grades to pull. Did not come out of 10th gear, unless turning .
With the 4 tanks, hate to fill it up. I only run it part time. And will be a while before I fill it up. Thinking about shutting off three tanks. Then fill one, but hate to let 400+ gallons set around in the tanks. 1995 KW T600.
I know we got unreal MPG on the 11.1's around 93 to 95. Ran From tn to LA,OR,then Miami and back to Tn each week. Almost pulling an empty trailer, both ways. And one Qtr we averaged 11.3 MPG. Trucks were set at 68 MPH, We ran team 6800-7200 miles per week.
The same Condo FLD120's with the same gears but 12.7's. high 7's low 8's same trips. We got a new truck every Qtr.

 

Scanguage my truck does not have the Detroit readout. Wish it did, figure the scangauge needs adjusting big time. But do know the 11.1's almost empty goes get good MPG's.