That 10 speed likely helps as does the gear ratio.. I was under the impression that the jump from a 5.3 to 6.2 was just about the same anyway depending on how you drive it.

Fuelly.com shows an average of 15.5 across 12 trucks with the 6.2 with a total of 153k miles so your are smoking the competition.

 

6.2 doesn't have to work as hard so the load is less on the motor.

 

On the highway with mine I average between 22 and 26 mpg.
around town its between 18 and 20.
I agree the ten speed helps with the mileage.

 

Motor has more power .trans has 4 more gears more gears in a trans more consistent power better MPGS 😎👍

 

Not to mention six speeds were plagued by faulty trans thermostat 😂

 

Yeah, on a good calm day my 5.3 w 6 sp would get 20 maybe 21 & I was happy. I did disable afm & it didn't change mileage. Now I've gotten almost 24 mpg on perfect days & everything is stock. Really love the combination of power & mileage of the 6.2/10 speed.

 

Just purchased a 2019 Sierra Denali....great to see the mileage you're getting. Are you using premium fuel?

 

I traded in my 2005 GMC Sierra 1500 w/5.3 for my 2018 GMC Sierra 1500 w/6.2. On the highway w/the 8 speed transmission I can get ~ 22 mpg if I keep it around 65 mph or slower. That is a very large increase over my 2005 GMC. I got 17~18 mpg on the highway even if I was very conservative in my driving. I did deactivate the AFM long ago. However, when I first had my new truck I watched the V8/V4 switch. My truck rarely switched to V4. I literally had to be coasting on level highway or going down hill letting the engine slow me down. Even touching the accelerator facilitated a shift back to V8. Additionally, unlike some I have read about I could not detect a change from V8 to V4 (or back again). No change in feel or sound...none...could be I'm just deaf as a post and lack feeling in my rear though. I deactivated the AFM because of reported lifter problems. At the time I became aware of the lifter issue most sources reported that some vehicles with GM's AFM had lifter/cam issues after 100K to 200K miles. Since then the lifter issue has become a much more pervasive, current problem.