Now that I've run it a few weeks, I think that if I had to do it again I'd go with 3.73s. I just completed a 2500ish mile trip, and am glad to say that on the trip I was able to best my own personal 20.4MPG with 20.5MPG. As I expected it does indeed stay in AFM more than it was and it's having a far easier time with hills. The shifting is going to need some work because it's not downshifting like it should. I've also noticed that AFM is more noticeable when it engages. The feel is no different but it's far more audible now than is was. Any more projects I decide to do, I'll start a new thread.

Thanks for the update.

1)How much did your cruise rpm increase?
2)Stock exhaust?
3) What kinda tire height are you running?

I wonder if this was a non-Direct IUnjection engine like in the GMT-800's/900's if the swap to 3.73 would still leave you short on the off idle/midrpm torque'ish feeling? The L83 5.3/L86 6.2, heck even the 4.3 LV1 are strong in that area.

When I went from 3.08s to 4.10s in my newish 1997 rcsb GMT-400 L31 350. It went from a 1700rpm- wouldn't hold OD/lockup on a grade a 70mph, to a held 4th gear/TCC lockup at 65mph with the cruise engaged on the same exact grade. I would "breeze" my right foot on the accelerator and the cruise control motor would barely even command more throttle to climb the grade. With the 3.08s, I could feel the pedal drop by about an inch, then the TCC would unlock and the trans would drop from OD to 3rd trying to maintain its cruise speed on that hill.
The 4.10s were awesome for city, 50mph driving and for quarter mile racing as I would trap in 3rd gear at about 4900rpm(stock rating was 255hp@4600rpm/330lb/ft torque@2800rpm). With the 3.08s I would trap at 49

However when late for work, or just attempting to emulate the "SPeeder", kicking the cruise speed up to 75mph resulted in an uncomfortable 2600rpm cruise rpm. With the 3.08s, that same 2600rpm could have me at the stock speed limiter at 98mph in OD locked up, but normal 75mph cruising was a much more relaxed 1900rpm.
I realize that describing a 2600rpm cruise rpm as "uncomfortable" sounds odd, esp. coming from a time when my first trucks came stock with 3 speed autos with no OD, but I also rocked 285.70/15(31" BFGs) and the 315/70/15 (33" BFG SPort Truck T/A H rated tires) in the older square bodies. I know you remember your Dad's or Brothers or your GrandDads(sorry cant remember) square body with the 400 SBC, IIRC it was a 1976 Chev???.

I loved those trucks as they had the "vent" style triangle windows that were perfect for ventilating the cabin. Just crack it a bit and it would suck you cigarette smoke right out and to ash, you just had to flick your smoke in the general vicinity of the vent window and aerodynamics would take over. Of course in the Winter up here, the occasional "snowdrift" would accumulate on the inside of the door if you didn't take extra care in properly closing the vent windows. Mine always leaked as they were old and well used.

peace
Hog

Unfortunately I didn't note the pre-gear RPM at cruise, but 80MPH it sits around 2350RPM, in that range. It's all factory on tire size (Crapyear Assurance, 265/65R18, 31.5 inch diameter) and all that though, so anyone else with a 2015 Tahoe with the 53L, 6 speed auto and 3.08s would be able to say what my pre-swap RPM was. I'm thinking it was around 1900RPM at 80MPH before though.

It's definitely going to need some tuning work because it gets better mileage at 65 to 70 AND 85 to 95 (had the opportunity to follow someone running 95 or so for a while) than it gets at 80MPH, per the instant read gauge. Speed vs mileage is way too inconsistent on where it should be, so I'm thinking that GM tuned specific fields only then used some sort of software guesswork to fill in the rest. Of course, I could be talking out of my backside here.

I don't feel like I'm short on power for what the engine is but I do still feel like it's just too little engine for the mass it has to move. Having my fat backside in there doesn't make it any better either. I've set a weight loss goal for the Tahoe of 400lbs, going to see if I can make it a little easier for it to move the mass. There's a lot of excess on there that I can pull off to get there. Will I make it? Who knows, but then a goal that is easily achieved is not worth making.

I definitely remember the box trucks. My first truck was a 1979 GMC, which I dumped for a 99 model (one of my huge regrets in life) and I miss those vent windows too. I also miss the floor vents, pull the little knob and you had airflow around your feet. With those vents and the wing windows you didn't need air conditioning in the truck except for those very few days when it was 90 degrees and raining.