My 2016 Sierra with a 5.3 would average about 16-17 in the city and 22-28 on the highway.Interesting...I've owned a 5.3 equipped vehicle for 10 years now and NEVER have seen over 20 and I drive like a little old lady most the time. I did a 1K mile round trip all highway when we first bought our '21 and both hand calculating and the computer came back at 18.4 mpg
One time I got a best of 31mpg, but I was going about 55-60 on flat ground.