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Wheel offset will affect the biggest tire size, a 34" will rub on full turn. Start cranking the torsion bars and playing with wheel offset and things change, but I learned my lesson years ago and leave it alone and save my front end.
 

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Well said by MORSNO. To add to his response ... GM has maximized the amount of tire that will fit in the fender well at full turn when the suspension fully compresses. You can lift the front end by cranking up the torsion bars, but when you start cranking the torsion bars you change the angles on the CV joints, upper & lower ball joints, and the tie rods as well. Many people have lifted the front end by cranking up the torsion bars 2 inches on 2020’s to fit 35 inch tires, but this noticeably stiffened the ride. It also moves the upper control arm closer to the stop, so when the front suspension drops it hits the stop after only an inch or less of travel. This is where the leveling kits come in, many cheaper kits replace the upper control arm with one that is more arched to allow factory travel without bottoming out. However, this does nothing for the angles put on the upper & lower control arms, CV joints, ties rods, etc. So some guys purchase the more expensive leveling kits that include new knuckles (and upper control arms). This corrects the upper & lower ball joint angles and tie rod angles, but there is still a steeper angle on the CV joints and the stiff ride caused by turning up the torsion bars. All these steep angles cause premature wear on parts as MORSNO was alluding to. As you can probably tell, I been down this road on my 2019. I will never again install a leveling kit or crank up my torsion bars. So if you want bigger tires and want to do it right without sacrificing a nice ride and causing premature wear on parts, you are into a differential drop lift kit like I finally did. This lowers the front differential and returns all stock parts to the proper working angles. However, $3k for a differential lift kit, $3k for wheels & tires, $4k to re-gear the differentials, $2k to beef up the steering components for bigger tires, $1k for a tuner w/ display to correct the speedometer (which you should do if you change any tire size) and to fix all the error diagnostic codes that come up re-gearing differentials on an L5P. So you are $13k to do a lift kit right, actually, I am closer to $15k when it is said and done ... last step is re-gearing my differentials over Christmas. So was it worth it ... yes and no. The yes ... my truck actually rides better with bigger 35 inch tires and properly tuned shocks, but I also added Sulastic shackles to take the jarring hits out of my back end when I hit potholes or sharp dips in the road. Plus my truck looks sooo much better than stock, the tire size actually looks proportional to the truck size instead of those little factory donuts. The no ... it was extremely expensive to do a lift kit right on an L5P ... which you cannot even accomplish right now with a 2020 because there is no tuner developed yet for a 2020 that allows for differential re-gearing or speedometer correction. If it was me, I would put a nice set of wheels and stock size tires on, buy some lowering shackles to level the backend, and install a set of air bags so the backend does not sag when you haul or tow something heavy. Then start saving your money to do it right with a differential drop lift kit. Not trying to be a “know it all” nor cast a negative shadow on installing bigger tires ... just know I recently went down this exact road and it ended up being a very expensive lesson to get my truck back to having factory power and a similar ride ... and it all started with bigger tires.

Side Note: Regarding installing the lower differential gears over Christmas to get my factory power back with 35 inch tires ... I have lost an entire gear towing on the highway until I change the differential gears (it is like having a 5-speed now). Plus my truck lost significant bottom end speed and the transmission shift points are also wrong until I re-gear the differentials. Hopefully this is the last item needed on an expensive lift kit build ... however I can say it was done right.
 

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Wheel offset will affect the biggest tire size, a 34" will rub on full turn. Start cranking the torsion bars and playing with wheel offset and things change, but I learned my lesson years ago and leave it alone and save my front end.
not correct. some 2020 trucks (those with 20" wheels) come with a 34" tire (275/65-20) from the factory
 

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not correct. some 2020 trucks (those with 20" wheels) come with a 34" tire (275/65-20) from the factory
You are correct ...some come stock with 34 inch tires ... 275/65R20 to be specific ... 34.1 inches

1082678

1082679
 

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Juat picked up my '21 3500HD AT4 yesterday and it has the 275/65/20's on it. I don't want to mess with my suspension at all. THerefore, I am only thinking of installing slightly larger tires. Will 295/65/20's fit? I am looking at a Ridge Grappler by Nitto. If not, surely a 285/65/20 will fit?
 

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Juat picked up my '21 3500HD AT4 yesterday and it has the 275/65/20's on it. I don't want to mess with my suspension at all. THerefore, I am only thinking of installing slightly larger tires. Will 295/65/20's fit? I am looking at a Ridge Grappler by Nitto. If not, surely a 285/65/20 will fit?
from all the research I have done, the 295/65 should fit. guys say they are running 35/12.5 stock no problem with no rubbing or trimming.
the 35 ends up being about 1/2" shorter, but about 3/4" wider than the 295
 

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from all the research I have done, the 295/65 should fit. guys say they are running 35/12.5 stock no problem with no rubbing or trimming.
the 35 ends up being about 1/2" shorter, but about 3/4" wider than the 295
Any issue with the 3/4" wider tire in the 35/12.5? Just curious about the upper control arm and if the 35/12.5 will rub. I could always install a 35/11.50 or 285/65/20 as well.
 

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I ran 295/65-20 Defenders on stock wheels, no level and no issues.
Currently running same tire on 20x9+20 Fuels. Again no mods and no rubbing
 

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21 GMC 2500HD Sierra Denali 6.6L Duramax CC SB "on order"
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Well said by MORSNO. To add to his response ... GM has maximized the amount of tire that will fit in the fender well at full turn when the suspension fully compresses. You can lift the front end by cranking up the torsion bars, but when you start cranking the torsion bars you change the angles on the CV joints, upper & lower ball joints, and the tie rods as well. Many people have lifted the front end by cranking up the torsion bars 2 inches on 2020’s to fit 35 inch tires, but this noticeably stiffened the ride. It also moves the upper control arm closer to the stop, so when the front suspension drops it hits the stop after only an inch or less of travel. This is where the leveling kits come in, many cheaper kits replace the upper control arm with one that is more arched to allow factory travel without bottoming out. However, this does nothing for the angles put on the upper & lower control arms, CV joints, ties rods, etc. So some guys purchase the more expensive leveling kits that include new knuckles (and upper control arms). This corrects the upper & lower ball joint angles and tie rod angles, but there is still a steeper angle on the CV joints and the stiff ride caused by turning up the torsion bars. All these steep angles cause premature wear on parts as MORSNO was alluding to. As you can probably tell, I been down this road on my 2019. I will never again install a leveling kit or crank up my torsion bars. So if you want bigger tires and want to do it right without sacrificing a nice ride and causing premature wear on parts, you are into a differential drop lift kit like I finally did. This lowers the front differential and returns all stock parts to the proper working angles. However, $3k for a differential lift kit, $3k for wheels & tires, $4k to re-gear the differentials, $2k to beef up the steering components for bigger tires, $1k for a tuner w/ display to correct the speedometer (which you should do if you change any tire size) and to fix all the error diagnostic codes that come up re-gearing differentials on an L5P. So you are $13k to do a lift kit right, actually, I am closer to $15k when it is said and done ... last step is re-gearing my differentials over Christmas. So was it worth it ... yes and no. The yes ... my truck actually rides better with bigger 35 inch tires and properly tuned shocks, but I also added Sulastic shackles to take the jarring hits out of my back end when I hit potholes or sharp dips in the road. Plus my truck looks sooo much better than stock, the tire size actually looks proportional to the truck size instead of those little factory donuts. The no ... it was extremely expensive to do a lift kit right on an L5P ... which you cannot even accomplish right now with a 2020 because there is no tuner developed yet for a 2020 that allows for differential re-gearing or speedometer correction. If it was me, I would put a nice set of wheels and stock size tires on, buy some lowering shackles to level the backend, and install a set of air bags so the backend does not sag when you haul or tow something heavy. Then start saving your money to do it right with a differential drop lift kit. Not trying to be a “know it all” nor cast a negative shadow on installing bigger tires ... just know I recently went down this exact road and it ended up being a very expensive lesson to get my truck back to having factory power and a similar ride ... and it all started with bigger tires.

Side Note: Regarding installing the lower differential gears over Christmas to get my factory power back with 35 inch tires ... I have lost an entire gear towing on the highway until I change the differential gears (it is like having a 5-speed now). Plus my truck lost significant bottom end speed and the transmission shift points are also wrong until I re-gear the differentials. Hopefully this is the last item needed on an expensive lift kit build ... however I can say it was done right.
Great read man. After reading everything you got I have a couple questions for you and maybe you can help a little.

I'm getting the 2021 GMC 2500HD Sierra Denali 6.6L Duramax with the 10 speed Allison. This will be my DD and I'll probably tow a 7.5k lb Trailer 5k-10k miles a year.

What I'm getting installed:
6.5" BDS Lift Kit with Fox 2.0 Shocks
BDS (or another band) Recoil Traction Bars
Air Bags w/ air lift 2" spacer
35x12.5 Nitto on 22" Moto's

I've asked the guy who runs the shop about any additional upgrades I need to keep the ride performance and longevity of the truck.
He said "since your truck is brand new there no need to buy a new upper control arm at this time and he can't think of anything additional I'll need at this time.

I asked about rear axle gear ratio since my truck is 3.42 he said "You have enough torque it shouldn’t need gears but yes we should do a speedometer recalibrator like super chips so speedo reads correct and tranny shifts properly"

After reading your post I'm wondering if I need to get the 2k in steering components upgrades..
 
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