Heres some facts from my experience.
This is on my ‘17 half ton crew cab 4x4, not my power wagon. But i have kept track and recorded every single tank of gas I have put in it since brand new. 49 fillups so far. I have written down what the evic said and what the actual calculation was. These are the average for a full tankfull, not partial or several tankfulls. The furthest off it has ever been was 1.83 mpg’s, while the closest it has ever been was .05 mpg’s. Both with the evic reading higher mpg’s than the actual hand calculation (imagine that! Lol).
The average the evic was off for the 49 fillups was .9631 mpg’s.
Also, best hand calculated average for a tankfull was 20.58 mpg’s, while the average for the last 49 fillups was 17.9804. NOT TOO BAD!!!!