I calculated my millage by hand for the first 6th months and the trip computer was never more then 1 mpg off, so its accurate enough.
Best I ever got for a whole tank was 27.3 mpg. That was on 95 between Washington DC and NYC, 700 lbs of people, dogs, and gear in the car. 65-75 mph average speed, but with some light traffic in places. I am pretty impressed by that.
Now all city driving, I can get below 20 sometimes, but its all short trips (less then 3 miles), so not ideal conditions for good mpg.