Yeah, so my wife and I have been messing around with it and today we took a 10 mile trip on the Northern state toward Huntington and back (since you are clearly a fellow LI guy), computer calculated average MPG at nearly 11 mpg up from 9.8mpg just from about 20 minutes of highway driving. I think we figured out the issue. When my wife picks up the kids from school this year, she has to sit in a car line for up to 30 minutes and while the auto-shutoff kicks in for a while, since the A/C is running, the engine actually runs most of that time. So I think it's the idling that's killing the mpg. We have this 1,300 mile trip planned about a month from now and I am going to get the oil changed and everything topped up before then. We will see what 1,300 miles of I-95 driving does. I will report back my findings.
You are definitely idling this thing a ton or just sitting in traffic with it. You stated earlier 160 hours at 1,500 miles. Just think about that math, you are averaging under 10 miles per hour of engine use. That is incredibly low and possibly low to the point of causing problems with the engine. They are not designed to just sit at idle. For comparison, mine ('21 rst) has 200 hours on the engine and I am just over 7k miles. This includes plenty of city driving, winter warm ups with remote start, and daily car line pickup for the kids at two different schools.
I have also been disappointed with mileage but that is at 14-15, not 8. I started keeping the little mileage thing up on the information display and, by watching what it likes and doesn't as I drive, have been able to get the same trips up to 20 and extended driving under ideal speeds up to 26. A couple of things I have found to help:
Coast into your stops. My habit previously was gas or brake, rarely neither. Coast into slower speed limit zones, coast half your speed off when coming up to a stop sign/light, coast when slowing for a vehicle ahead of you traveling at a lower speed. This one makes a really big difference.
Don't sit in park with the engine running unless you are warming it up. The 5.3 guzzles gas at idle and this vehicle is surprisingly good at holding interior temp with the engine off so what is the point? This also means don't disable the autostop. That feature really makes a difference around town and it is pretty easy to adjust to.
This one might surprise some, but I get better mileage with a heavier foot. If I accelerate slowly up to my desired speed I actually get worse mileage vs accelerating with authority up to the same speed. Not flooring it, but maybe 1/2 throttle or a bit below. Plus it is fun and the engine sounds great when driven like that.
Again, this is with the same truck, 2021 rst with the 5.3, although I took delivery in November so maybe they are building them different now due to the chip shortage? FWIW, I just got our 8k pound boat out of storage and towed it 35 miles into a slight headwind that included two towns worth of city driving. I averaged just over 10mpg.
