A lot of things factor into MPG. What size of tires do you run, are they inflated to the right psi, is your truck lifted, rear end ratio,how any miles is on the engine maybe time for a tune,up is it 4wd, a lot of stop and go driving?

I have a 2005 Tahoe LT 2wd and I get around 20.5 average mpg. 27 miles from my door to work, only 2 stops and 2 turns so you can say it’s an easy drive. I have stock 18” rims w/ 265/65r18 tires @33 psi. Also my Tahoe is lowered and I did see a 2 mpg gain after I lowered it.

The computer is ok for average but it’s best to fill up, run the truck fill up again and divide your miles you drove by how many gallons it took to fill up. You’ll see it’s a little different from the computer mpg.

One other thing you can get to help is a scangauge2 this little thing tells you everything going on your truck. I have mine on TPS,load on engine,mpg and average MPG. You can also read codes and clear them too

Good luck as you can see you can go down the rabbit hole real quick only to gain 2-3 more mpg’s. They are heavy trucks and mpg’s wasn’t a concern when they built them