Getting Ready for 37's on 2020 Silverado LT Z71

Txgunslinger · July 4, 2022

So I've been researching and shopping for parts for a few months now and here's where I'm at currently:

- BDS 6" coil-over kit

- 37x11.50x20 Nitto Ridge Grapplers

- Black Rhino KAIZEN (20x9.5 on 6x5.5 / 6x139.7 bolt pattern, +12 offset, 112.10 bore) Dearborn Blue

Still to come:

- gears (3.90? 4.10? 4.56?)

- tuner (looking at Diablo flavor but open to suggestions)

- headers (would like 1 3/4" LT to help low-end, but open to suggestions. Stainless Works or Kooks likely)

Yes, I'm going with 'pizza cutters' as I'm more interested in ground clearance for the occasional off-road jaunt and hazardous water crossings. It's still mostly a highway truck to get me to storms and hurricane zones and of course handling claims, so definitely a daily driver (I just turned 53K while working up here in MN). The truck came with an 8-speed trans and 3.23 gears on stock 265/65x18 tires. By my calculations, the 37's are 17.0886% larger than stock (31.6 inches) and applying those calculations to new gears would be 3.78 or so (3.73 or 3.90).

I was originally thinking 4.88's and then 4.56 to allow me to downgrade to 35's in the future, but I'm not towing anything (yet). Given how much time I spend on the road, I am concerned about mpg and would like to maintain as much as possible. The 17% came from https://www.4wheelparts.com/the-dirt/a-guide-to-gearing/ and 3.73 or 3.90 surprised me and has me wondering. I realize I'm going to lose some grunt by not going with higher gears, but I can't afford to tank my gas mileage (pun intended).

If I went with 3.90 gears per the chart, I'm still in the 'green', but I feel like those RPM numbers don't take into account how many gears you have in your tranny nor any shift point tuning. Are the RPMs fixed like that regardless of anything else?

I also realize gearing low puts additional strain on the drivetrain/trans so I'm trying to find a happy medium if possible. Thanks for reading and your input folks. Pics to come once I get home and get to start putting this stuff on. Pic shows my stock truck with a headache rack that I just removed and replaced with an ADARAC Pro kit, but the big changes are coming. Thanks for your input!

matienzone · July 5, 2022

I’m definitely down to follow this thread. 37s on our trucks is pretty rare from what I’ve seen. Good luck trying to find the happy medium fo ryour final drive. I took a look at the chart, and I’m voting for 3.90. Seems like the balance between perform every and the gas mileage you’re looking for. Still some extra room if you want to go 35s in the future and it would just get peppier. Good luck with the build!

AJMBLAZER · July 5, 2022

Some of the old metrics for gear ratios and tire sizes went out the window when the 6, 8, and 10 speed transmissions came out. Low 1st and 2nd gears with more reduction than the old granny low manuals in 1st as well as 2 or 3 over drive gears means we don't have to do all the ratio lowering in the axles any more.

I'd say 3.90's if you want more mileage and 4.11's if you want more grunt.

Recalibrating the speedo/computer?

Txgunslinger · July 6, 2022

6 hours ago, AJMBLAZER said:

I'd say 3.90's if you want more mileage and 4.11's if you want more grunt.

Recalibrating the speedo/computer?

Yours is the second response that includes 3.90 as a possible choice and probably what I’m leaning towards. I figure it’ll be better than the stock 3.23’s and still keep the RPM’s at a mere roar. And yes, recalibrating via the tuner I still plan to get (most likely some flavor of Diablo). With all the driving I do, I can’t afford tickets so knowing my correct speed is critical.

AJMBLAZER · July 7, 2022

Does Diablo have a tuner for the 19+ trucks yet?

CamGTP · July 7, 2022

37's and 4.88 would be ideal for a 4 speed auto, not so much an 8 speed. My opinion says 3.73's or 3.90's but what is even out there for these differentials. Can you even get 3.90's for the front and rear? Last I checked you can't get them for the front but things could have changed. Going to 4.10's or higher will give more pull down low but push cruising rpm higher.

Don't install headers if you want to keep all of the low end grunt, headers will just shift power band up. Not that it's a drastic difference but it does move it up.

I work in the automotive and tire biz, I'd suggest not running the Ridge Grappler but that's just me. Look into the Recon Grappler instead if you are wanting Nitto's. The tread pattern and sipes are better designed to handle lots of water and up here in the north, would handle the snow better. The Ridge Grapplers are pretty bad in the really cold months and because of the big block tread design aren't very good in the snow.

Keep in mind, the bigger the tire the bigger those tread blocks get. And on 37's, they are pretty big.

Edited July 7, 2022 by CamGTP

clowe1243 · July 8, 2022

I'm running the 6" BDS coilover kit too, which is actually yielding 6.5". Running 37s on a 10-speed however and which I believe is 3.23. Have seen no issues from a dig and or highway passing.

In regards to the tuner, I currently running a Pulsar LT (Not really a tuner); and waiting for a moment to be without my truck, to send my ECU in to Superchips (Holly) for mod'ing. Already have the SC tuner.

All together with 37s, lift, AFM disabled, start-stop disabled, I average 18-20 highway, 15-17 city, DC traffic sometimes as low as 9mpg...smh.

And here's a picture just because I love my truck!

Gangly · July 8, 2022

By my calculations, going from a 33" tire with a 3.23 ratio to a 37" tire would require 3.62 gearing to maintain that "factory performance feeling". Though a difference in ratio, its not a huge difference and I personally wouldn't worry about it. As clowe1243 mentioned above, with 8 and 10 speed transmissions you really aren't loosing much gearing.

Gangly · July 8, 2022



Final Gearing Ratio
10 Spd Auto	Gearing (X:1)	4 Hi Final Ratios	4 Lo Final Ratios	Tire Diameter Calculator
1	4.7	15.18	40.84	Width (mm)	275
2	2.99	9.66	25.98	Height Ratio (%)	60
3	2.15	6.94	18.68	Wheel Diameter (inches)	20
4	1.8	5.81	15.64	Total Diameter (inches)	32.99
5	1.52	4.91	13.21
6	1.28	4.13	11.12	Gear Ratio Needed after Tire install
7	1	3.23	8.69	New Diameter (inches)	37
8	0.85	2.75	7.39	Old Diamteter (inches)	33
9	0.69	2.23	6.00	Stock Rear Gear Ratio	3.23
10	0.64	2.07	5.56	New Gear Ratio Needed	3.62
Reverse	4.87	15.73	42.31

Axle Ratio	3.23	Crawl Gear Ratio (X:1)
		40.84
Transfer Case
4 Hi	1
4 Lo	2.69

dieselfan1 · July 9, 2022

Put it in tow haul and roll it.

I ran 38s with 10.5" lift on my 18 5.3 6 spd 3.42.

Superchips tuner and Borla catback helped a little too.

If that had enough balls to roll out, and it did you just had to give it more gas, your truck will be fine. Run it.

AJMBLAZER · July 9, 2022

Gears are the only way to return mechanical advantage.

Increased power is a bandaid for the lost leverage due to the bigger tires.

Txgunslinger · July 9, 2022

Badass information fellas, thanks. Can’t wait to get home and start putting this together. @clowe1243 beautiful truck and you’ve given my mpg hope

or at least put off the re-gearing for a little. So much money going out! Anyone running headers yet and did it help power wise? Gearing is supposed to cost me $3-4K depending on lockers, etc. and about that much for an LT header kit with cat back from Stainless Works. Guess I just need to spread things out, but probably proceed with gearing first. Maybe? So many forks…

Txgunslinger · November 30, 2022

Okay, so I'm finally back from Florida and got the lift installed. I called an audible at the shop and had them add some AMP power steps so mama can get in easier. Plus I like how they tuck out of the way while driving. The tires don't stick out that much so that's cool and they don't rub either so the rim offset is good. Rims are a bit flashy for my taste, but the color is pretty nice. I'm just glad to finally have some ground clearance and something that looks a little different than most of the other lifted trucks out there. Can't afford to regear at the moment, but looking at tuners and thinking DiabloSport Predator 2 (7222-R) to adjust some shift points and disable DFM. I can feel/see the transmission hunting for gears a little and my gas mileage just took a nose dive lol. I was averaging 17-18 and right now I'm down to 10-12 mpg. Guess that's the price I pay to look cool lol. Going to the body shop to get a new tail gate soon (not my fault) so I'll get some better pics after she's all fixed up

Amcguy1970 · November 30, 2022

15 hours ago, Txgunslinger said:

Colors are a bit flashy for my taste, but the rim is pretty nice.

Fixed that for you.

Is it me/the angle or is that squatting? It looks quite nose high in the above pic. Did you adjust your headlights down?

Tyler

Txgunslinger · December 2, 2022

On 11/30/2022 at 4:56 PM, Amcguy1970 said:

Fixed that for you.

Is it me/the angle or is that squatting? It looks quite nose high in the above pic. Did you adjust your headlights down?

Tyler

lol thx. It's just the angle of the photo and the fact my street slopes. I'll get some better photos after I get it back from the body shop, but I think it actually sits pretty level, definitely more so when it was ass high in the stock stance.