duramax Diesel question

I can concur that the DICs are not always accurate. Some people claim theirs are very accurate however on my 2012 I compared the DIC to hand calculated for the first 15-20 fillups and confirmed the DIC reads about 5% high on MPG figure. The fuel consumed was always less than what I was able to refill the tank with by about 0.5-1gal on every fillup resulting in higher MPG figures. Now that I have characterized how much my DIC is off by I can still use the DIC and subtract ~5% from the MPG figure. And yes when they are off they typically read high. I have yet to see anyone say their DIC MPG was lower than hand calculated.