I am getting a pretty solid 16.5 MPG combined in my '13 YXL Denali with the 6.2L.
Only 1700 miles so far but soon I will switch to Mobile 1 oil and if it goes like it has in the past, I might see as much and 0.5 to 1 MPG more.
I also run about 45 PSI in my tires, and that helps.
What Jennifer T says is right. There are so many things that can impact fuel economy that people don't think of, or are not aware of.
A big part of my job is evaluating fuel economy.
One of the biggest things I have seen impact FE is switching to larger diameter wheels. A big thing that people are not aware of is that when you accelerate a vehicle, you not only need energy to change the speed of the vehicle, but you also need energy to spin up the wheels. The heavier the wheels are, and, the larger the diameter, the more energy is required. In fact, the Polar Moment of Inertia of the wheels is a function of the square of the radius. So, a small diameter increase of the mass can result in a large increase in the Polar Moment of Inertia.
Think of the wheels/tires as flywheels. The heavier they are, and the more mass concentrated further out, the harder it is to accelerate and decelerate them.
I recently tested a vehicle (car) where all we did was change from 17" wheels to 20" wheels, with the rolling radius very nearly the same, and we saw a decrease in fuel economy of about 20%. Not only where the wheels heavier at about 62 lb, vs 37 lb, but that extra mass was distributed at a bigger diameter.
Also, the 20" tires were inherently less efficient.
Tires have an efficiency rating (usually not published) that relates the energy it takes to roll the tire under load (basically, how the energy require to make the tire continuously deform its shape as it rolls around.
In general, the OEM selects tires with a pretty good efficiency rating since they have to run those tires when the official EPA MPG numbers are determined. However, by far, most aftermarket tires, whether original size or not, have a lower efficiency number. Sometimes much lower, and that can cause a big FE reduction.
Since these numbers are not generally published, it can be very hard to tell what to select. You can always try to go with the OEM tires to preserve the FE but if your like me, that is not a very desirable option
And when it comes to tires that are much, much different than the OEM setup, like off road tires or 26" tires and such, all bets are off. You can easily drop 30-40% on you FE.
