15MPG normal for 2014 5.3 Denali?

[L86 All Terrain]

You probably have 3.08 gears in your truck. I put 33" 10-ply on my 2010 5.3L with 3.08's and it became a dog and gas pig at the same time. I have a 6.2L on 33 10-plys and now I have a powerful gas pig. I get about 18mpg combined with my 6.2L 8spd, I was around 16mpg with the 5.3L 6spd. My 2014 WT 5.3L 3.42gears has 31" 10 plys (on a lighter 17" wheel) and gets 19-20. 

 

Guys on here saying these trucks get 23-25mpg with a 33" only travel downhill with a tailwind, or they are blowing smoke up your butt. Took my 6.2L to the city yesterday and got 16.2mpg on the flats trough the prairies with little wind at 76mph, when I slowed it down in Ontario in the rolling hills and single lane highway I got 18.8mpg at 67mph. That is just DIC readings too, so you can subtract 1mpg off that minimum. 

 

My truck does best below 50mph, cruising the slower secondary highways I can get 20mpg but that is hard to achieve and you are really going for it, slowing down halfway up the hills, coasting fully down the other side, not passing people etc.