barr8888

(digging up an older thread here) This is likely not news to anyone that’s been searching, but the latest information I have (this time from East Coast Gear Supply in NC) is that re-gearing options are still not available for the front differential in these trucks. The reason cited was once again related to how the OEM ring gear is attached to the carrier (I’ll avoid using the term “welded” this time, though I’ve now heard it described that way from multiple sources). If anyone has had better luck, do chime in. 2022 Limited AT4 here, with SU5 GM front axle / SU7 AAM rear axle.

Mine cut off, too. I presume they all do that, for whatever reason. Ask your dealer's service dept. (any dealer should be able to do it) to send you a copy of your truck's build sheet. All they should need is your VIN, which your own dealer may already have on file. The build sheet will list all of your RPO codes (mine is 6 pages long) and a brief description for each.

As has been stated, there's really nothing to be gained from removing the front cover that you can't more easily determine from your RPO build codes and VIN. My thoughts back then were related to some apparent rumors/misinformation related to the front ring gear having been attached to the front carrier by GM in such a way that the entire carrier would need to be replaced to change the gearing, instead of just changing the ring and pinion gears themselves, as one would expect. I went around to some driveline and performance shops at the time and no one had any definitive information (including supposedly knowledgeable GM industry contacts) on what, if anything, was available for the front axle in '19+ GM trucks. The rear axle (RPO codes listed further below) apparently has aftermarket gear sets available, regardless of axle manufacturer (AAM or GM). It's the front axle where all the difficulty comes into play, as I'm sure you know. I basically gave up back then and decided to wait until the pandemic's effects on industry were a little further behind us. (As a side note, I still intend to replace the rear OEM limited-slip carrier in the rear with an Eaton TrueTrac, as there's no better time to do it than during a gear set change. I believe the TrueTrac will be more robust/reliable than the OEM solution, especially with the increased mass and leverage of the larger tires). The ideal (and likely most cost-effective) solution is an aftermarket gear set for the front axle, but this depends on whether or not any are available. If not, then this is where RPO build codes and VINs could come into play. First, you need to know which front axle you have, an AAM or a GM (mine is a GM, RPO=SU5). Then, you need to determine what ratios GM uses (or used) in that front axle. If they used a 3.73 in some truck combination, then you might be able to order GM parts that will accomplish what you need, if the parts are available, and if you can definitively determine the correct part numbers (this is where the VIN from a truck that has the front axle and ratio you need can prove invaluable). From my research back then, it looks like the 3.0 Duramax diesel 4x4 truck was available with 3.73 gearing. What I was never able to determine, though, was which front axle GM uses (or used) in that truck. That's where my own research into a potential OEM 3.73 solution stalled, so I never got to the part about whether these parts are available/backordered/cost/etc. '22-Limited GMC AT4 Front axle: SU5=GM, SU4=AAM Rear axle: SU8=GM, SU7=AAM Do keep us informed of any progress you make. It's probably time for me to resume my own research, too. The 6.2L, 10-spd trans, and tow/haul do a good job of masking the gearing issue with my ~5k trailer, but I can still tell it's not ideal. And even in no-trailer highway cruising scenarios (w/out tow/haul engaged) the slight surge/sluggishness at times is definitely noticeable, as the trans ratios were never designed for an (effectively) 3.08 axle ratio.

I can't say for sure if it's tire related, but specific tire RPO codes were listed in the GM bulletin I was given when I asked about the inaccessible 'Sport Mode' on my '22 Limited AT4. GM Bulletin #PIT5871 Unable To Select Sport Mode Document ID: 5929555

It's not my intent to spread misinformation... I'm only citing what I've personally been told by a GM dealership parts person... and have also read elsewhere. I acknowledge that it doesn't sound like something that makes any sense to me. I don't see how the hardness of the ring gear itself could be maintained if any attempt was made to weld it, for one. I haven't taken the cover off my front axle to verify anything personally yet. But finding reliable information and/or parts to change the front axle ratio has been much more of a challenge than I was expecting, so far.

I'm in basically the same boat. See this post in a different thread: I'm actually in slightly worse shape, since my AT4 has factory 3.23 gearing. I'm at (effectively) a 3.08 gear right now with 35" tires and the stock 3.23's. The 6.2L and 10-speed trans DO make up for a lot, but 3.73's would get me pretty close to (effectively) stock 3.42 gearing, which is the same thing you're after (and should be perfect for my needs). While I'm pretty sure rear axle ring/pinion sets are available aftermarket (my rear axle is an SU7; an AAM axle. SU8 is the GM rear axle)... the big questions revolve around the front axle. Rumor has it that GM may be welding (or otherwise permanently attaching) the ring gear to the carrier in the front axles on these trucks. If that's true (and I've heard it from more than one source) then that may be part of the reason the aftermarket doesn't seem to have any ring/pinion sets available (yet, anyway). Now I'm not opposed to purchasing a whole front carrier (w/ring) and pinion directly from GM in 3.73 if that's the only way to do it right now, but that brings additional questions into play. GM is using two different front axles on these trucks. The SU5 (what I have) is a GM front axle. And there's also an SU4, an AAM front axle. So, if the 3.73 front ratio is only available with the 3.0L Duramax diesel, which front axle are they putting in those trucks?... and/or... are the internals (ring/pinion/carrier) interchangeable between the AAM and GM axle? Because if ALL of the Duramax diesel 3.73 geared 4x4 trucks have AAM axles... and the internals are not interchangeable... then even trying to buy 3.73 front gearing direct from GM won't work for me (and my GM front axle). Just food for thought. If you come up with a workable solution, please do come back and post it here (I'll do the same).

Thanks. Oh man. I'm not sure if I want to add it all up. Probably 6 or 7 grand after everything. I do still have the complete OEM wheels/tires to sell (with about 300 miles total on them), so I can recoup a little bit of it there (if I ever get around to selling them). - Pro-Comp 4" Lift Kit was right at $2k - Lift kit install was around $1500 I had the lift kit installed by a commercial customizing place near me that's done a bunch of them on these trucks. That price included everything (new wheels/tires installed, having a dealership paint the flares to match, getting it aligned at a quality shop after install, etc.) - Fuel Siege wheels were $1600 - BF Goodrich 35x12.5R20 K02 tires were $1600 (4Wheelparts.com had a $100 discount on a set) ----------------------------------------------------------------------------------------------------------------- Not immediately noticeable (but contributing quite a bit to the overall look, I think) are the AMP retractable running boards. Definitely overpriced, imo, though I did get them to knock 200 off due to shipping delays that ended up being well beyond what they told me when I ordered. They ARE very nice, though. Well thought out, straightforward install, flawless operation, so far. Anyone considering should do themselves a favor and NOT order the AMP-branded OBD-II pass-through if they want to keep their OBD-II port accessible, though. Pick up the one from Auto Meter for less than half the price instead (part 5323, it's on Amazon for around $35). Bonus, that one actually bolts right into the original location, so your OBD-II port looks and functions exactly as it was before install. - AMP PowerStep Xtreme running boards $1800 Latest addition is a set of Air Lift helper air bags (89388) and wirelessly-controlled on-board compressor (25980EZ). The lift kit levels out the factory raised rear end of the truck, which is a much nicer look on a lifted truck, imo. But adding any significant weight to the rear (like that 7k GVW trailer sitting behind it) can potentially introduce some squat. The compressor has user-defined pressure presets for known loads (like a trailer). They run empty under normal conditions (5psi, just enough to keep their shape) and a touch of a button (or app control) pumps them up whenever I'm towing the trailer.

For what it's worth, the info below is what I gathered before changing to aftermarket wheels and 35" tires. While it's technically '19-'20 info, it also applied to my '22 Limited GMC Sierra AT4. 20 and 22" OEM wheels show the same +28mm (+1.1") offset, which equals a 6.10" backspace on a 9" width wheel. Also note the center bore at 78.1 mm, which will also need to match. Aftermarket wheel manufacturers tend to oversize the center bore to fit the largest number of vehicles, and often include several sets of adapters to reduce it to common OEM sizes. You'll need adapters (hub centering rings) if your new wheels are oversized and don't come with them. My Fuel wheels came with 3 sets of different sized adapters, but they're readily available otherwise ($15-$30 on Amazon). OEM wheel info 2019-2020 Chevrolet Silverado & GMC Sierra 1500 - 2wd & 4wd Factory 17x8.0 Wheel Offset = +24 mm - Back Space = 5.44" Inches Factory 18x8.5 Wheel Offset = +26 mm - Back Space = 5.77" Inches Factory 20x9.0 Wheel Offset = +28 mm - Back Space = 6.10" Inches Factory 22x9.0 Wheel Offset = +28 mm - Back Space = 6.10" Inches Lug Pattern = 6 x 139.7 Center Bore: 78.1 mm Bolt pattern: 6x139.7 (6x5.5) Thread Size: M14 x 1.5 ________________________________________________________________________ And not to hijack this thread, but for any potential questions from others regarding the 35" tire fitment I mentioned, below are the specific wheels, tires, etc., I went with. In my case, an approximately 2" wider tire (10.4" OEM to 12.5" BF Goodrich) with 1" less offset/backspacing in the wheel put the inner tread of the new tire in approximately the same position as the stock tire. The additional width to the outside (now 2" with the offset change) was covered by aftermarket fender flares (from EGR, in my case; painted to match). Finally, I'm using one of those Hypertech plug-in speedometer calibrator modules (730129T) to correct the speedometer. I've had zero issues with it, so far. OEM tires (spec height 32.5", spec width 10.4") 20" LT 265/60 R20 OEM wheel weight, complete assembly, minus lug nuts (actually weighed): 84 lbs. Aftermarket wheels: Fuel Siege Model D706 Matte Black 20x9 Size BP Part Number Finish Bore Offset backspace (in) 20x9 6x139.7 D70620908450 MATTE BLACK 106.10 +1mm 5.040 Tires: BF Goodrich All-Terrain T/A KO2 - Size: 35X12.5R20LT weight: 67 lbs. New tire/wheel combo weight, from specs: 102 lbs. OEM weight: 84 lbs. Difference: 18 lbs. GMC no-drill fender Flares: https://egrusa.com/EGR_Product_Pages/791794.php -Can be painted to match. -Black bolts available from EGR directly (call; $20) Hypertech 730129T https://www.summitracing.com/parts/HYP-730129T Programmed Scale factor I settled on after testing against GPS = 1.059 BEFORE/STOCK: AFTER ( includes 4" Pro-Comp lift kit K1176B )

This. 3.73 gearing looks to only be available in the Silverado with the 3.0L Duramax AND the 'Diesel Off-Road Package.' Can anyone that has the 3.73 gearing (I believe that's RPO code GT4) verify if their front axle RPO is SU5 (a GM axle) or are they all SU4 (an AAM axle)? I have a '22 Limited AT4 (SU5/GM front axle; SU7/AAM rear) that came with 3.23 gearing (on OEM 265/60/R20 tires). Now with 35X12.5R20LT tires, correcting back to stock gearing works out to about a 3.48 (3.42 being the closest available). But add in the extra unsprung rotating mass of the larger wheel/tire combo (around 18 lbs. at each corner), and slightly over-correcting to 3.73 gearing should be just about perfect for my needs. Since aftermarket gearing is apparently as yet still unavailable for the front axle in the T1 platform trucks (I'm hearing that GM might actually be welding the ring gear to the carrier in these axles, too). I'm trying to determine if GM OEM 3.73 front gearing will work in my SU5 front axle. Any help would be greatly appreciated. I don't believe the rear axle will be a problem, as aftermarket 3.73 gearing is apparently available there. And while it's apart for gearing, anyway, I also intend to replace the factory 'gov-lock' locking differential with a more reliable Eaton Detroit TrueTrac. I'm not so sure about ever coming down hard on a gov-lock with the added leverage 35" tall tires bring to the equation.