Axle ratio to tire size input needed

[SilverSurfer5300]

Hello! I'm new to the forum and I just purchased a 2013 Tahoe 4x4. It has the 6L80 6-speed tranny and the GU4 3.08 axle ratio.

I would like to replace the current 275/55/20s (31.9" diameter) with 275/60/20s (33.0" diameter), but all the online axle ratio-to-tire size charts show the current (OE) tire diameter to be too large for the 3.08 axle ratio, already. It seems that the majority of these charts are 20+ years old and were formulated with 3 and 4 speed transmissions in mind and thus, would be very inaccurate for determining ideal axle ratio-to-tire size relationships for 6 speed transmissions.

I am fine with changing gears in the front and rear axles if needed to put me in the ideal torque range. Does anyone have updated axle ratio-to-tire size charts specifically for 6-speed transmissions, or a way to [accurately] figure it out?

 

[Foggy]

Just google 6l80e gear ratios.. 5th and 6th are both OD ratios
You'll want at least 3.42 gears.. But with the 5.3 I'd personally go
with 3.73 . It's kinda pricey doing front and rear.
I have 3.42's with stock height tires.. I'd like a bit deeper like 3.73, but
I don't have experience rebuilding diffs and setting up gears.
My Yukon is used for towing
So You'll have to decide what your actual use is

 

[Marky Dissod]

... just purchased a 2013 Tahoe 4x4 ... 6L80E & 3.08.
I'd like to replace the current 275/55R20 (31.9" tall) with 275/60R20 (33.0" diameter), but all the online axle ratio-to-tire size charts show the current (OE) tire diameter to be too large for 3.08, already.

Because GM is held hostage by CAFE MPG test performance, this is absolutely true.
However, some vehicles are outside of CAFE's aegis.
Those vehicles, that also came with 6L80E, were equipped by GM with 3.42 or 3.73.

... I'm fine with changing front and rear axle gears if needed to put me in the ideal torque range.
Does anyone have updated axle ratio-to-tire size charts specifically for 6L80E, or a way to [accurately] figure it out?

Since GM undergeared your axles from go, instead of a calculator, answer these questions:
1. This is my toy, NOT my daily driver?
Yes, toy = 3.73 ... or ... No, daily driver = 3.42
2. I'm gonna race, or 'play hard' [for money]?
Yes, race = 3.73 ... or ... No = 3.42
3. I'm gonna tow/haul, or 'work hard' [for money]?
Yes, lots of towing / hauling =3.73 ... or ... No =3.42
4. I care more about city MpG, less about highway MpG?
Yes, city = 3.73 ... or ... No, highway = 3.42
5. Many or mostly steep &/or very long hills where I usually drive?
Yes, hilly = 3.73 ... or ... No, flatlands = 3.42

If N is greater than Y, 3.42. However, if N=5, keep your weak@$$ axle gears.
If Y is greater than N, 3.73. However, if Y=5, 4.10.

 

[Charlie207]

Did 3.73 gears come in any 4wd 6L80-equiped SUVs? Or, was that a truck-only thing?

 

[SilverSurfer5300]

answer these questions:
1. This is my toy, NOT my daily driver?
Yes, toy = 3.73 ... or ... No, daily driver = 3.42

No, daily driver. But I do plan on eventually increasing low-to-mid rpm torque ~50-100 lb./ft.

2. I'm gonna race, or 'play hard' [for money]?
Yes, race = 3.73 ... or ... No = 3.42

No.

3. I'm gonna tow/haul, or 'work hard' [for money]?
Yes, lots of towing / hauling =3.73 ... or ... No =3.42

Only occasional towing.

4. I care more about city MpG, less about highway MpG?
Yes, city = 3.73 ... or ... No, highway = 3.42

Yes, I care more about city mpg.

5. Many or mostly steep &/or very long hills where I usually drive?
Yes, hilly = 3.73 ... or ... No, flatlands = 3.42

Yes, hilly.

 

[Marky Dissod]

Did 3.73 come in any 4WD 6L80E-equipped SUVs?
Or, was that a pickup truck only thing?

Specifically, I don't think 3.73 was available for any '1500' GMT900 with 6L80E.
However, 3.73 was available for '2500' and up GMT900 with 6L90E.

Two more questions, by the way:
6. Will I be using taller- / heavier-than OE wheel & tire package?
Yes = 3.73 ... or ... No = 3.42

7. Did GM undergear every V8 vehicle they've ever sold since the late 60s?
YES.

Final decision is yours, of course, but you get the idea.

 

[blondie70]

Why not just run a smaller diameter tire/wheel and do same thing for much much less $$$$ ?

 

[wjburken]

@swathdiver has a pretty handy spreadsheet for this.

I made a calculator with excel that's very precise and even accounts for tread depth. Here's an example of my compact one:

View attachment 402949

This is about what the OP is experiencing now:

View attachment 402953

And this is how his truck would perform with 4.10 gears:

View attachment 402954

So if you compare the before and after, you guys were correct and I was wrong about the 4.56s being more equal, the 4.10s would be more equivalent while running 37s!

That's why I made this! LOL

 

[Charlie207]

Why not just run a smaller diameter tire/wheel and do same thing for much much less $$$$ ?

Because I have 33" wheel/tire combos for summer & winter.

I imagine a 6.0 swap will negate the taller tires.

 

[SilverSurfer5300]

Why not just run a smaller diameter tire/wheel and do same thing for much much less $$$$ ?

Because if I had wanted a minivan, I would have just bought a minivan instead of a Tahoe.


In all fairness, I know that the term "SUV" has evolved into a catchall term for everything that is neither a car nor a pickup, and the majority of current SUVs are basically nothing more than large hatchback cars with slightly taller suspensions than their sedan counterparts. . .

But I want my Tahoe to fit the traditional definition of an SUV: a 4x4 truck with a chassis-length cabin.

 

[swathdiver]

Hello! I'm new to the forum and I just purchased a 2013 Tahoe 4x4. It has the 6L80 6-speed tranny and the GU4 3.08 axle ratio.

I would like to replace the current 275/55/20s (31.9" diameter) with 275/60/20s (33.0" diameter), but all the online axle ratio-to-tire size charts show the current (OE) tire diameter to be too large for the 3.08 axle ratio, already. It seems that the majority of these charts are 20+ years old and were formulated with 3 and 4 speed transmissions in mind and thus, would be very inaccurate for determining ideal axle ratio-to-tire size relationships for 6 speed transmissions.

I am fine with changing gears in the front and rear axles if needed to put me in the ideal torque range. Does anyone have updated axle ratio-to-tire size charts specifically for 6-speed transmissions, or a way to [accurately] figure it out?

So what do you want to see, rpms at speed with a given tire and gear ratio?

Did 3.73 gears come in any 4wd 6L80-equiped SUVs? Or, was that a truck-only thing?

The 2500 Suburbans and Yukon XLs have 3.73s gears with the 6L90, for the purpose of this discussion it's the same thing, gearing is the same in the two transmissions.

Specifically, I don't think 3.73 was available for any '1500' GMT900 with 6L80E.

The NHT equipped GMT900 pickups with 6-speeds have 3.73 gears.

 

[Marky Dissod]

Specifically, I don't think 3.73 was available for any '1500' GMT900 with 6L80E.

NHT equipped GMT900 pickups with 6L80E have 3.73.

Dumb question:
So, GMT900 pickups with 6L80E got 3.73 ... what did GMT900 suvs with NHT get?[/QUOTE]

 

[vcode]

I guess, what is the real question here? It's not like your truck is suddenly not going to run. In fact I doubt you would notice any difference as you are only increasing tire size by an inch. 3.42's would more than compensate for the tire change though. I'd change the tires, drive it and see what you think. Junkyard 3.42 axles should be easy to find. Speedo will be off though, even if you just change tires.

 

[The_White_Car]

I guess, what is the real question here? It's not like your truck is suddenly not going to run. In fact I doubt you would notice any difference as you are only increasing tire size by an inch. 3.42's would more than compensate for the tire change though. I'd change the tires, drive it and see what you think. Junkyard 3.42 axles should be easy to find. Speedo will be off though, even if you just change tires.

Agreed. Just swap the tires and see. People run giant ass tires on stock gears all the time.

 

[Marky Dissod]

People run giant ass tires on stock gears all the time.

GM has undergeared every vehicle with a V8 since the mid '70s.
Running giant ass tires on an already undergeared vehicle is not doing the powertrain any favors.
And that fails to account for the unsprung weight gain.

 

[SilverSurfer5300]

So what do you want to see, rpms at speed with a given tire and gear ratio?

I guess, what is the real question here? It's not like your truck is suddenly not going to run.

I appreciate all the feedback so far -you guys have been very helpful and gave me the answer I needed. I didn't want to ask a super lengthy question, but since you're asking, here's the background:

I traded out of a bone-stock Nissan Armada that had approximately the same horsepower and only about 40 lb./ft. more torque than my Tahoe. It would overtake cars at highway speeds like they were standing still. The Tahoe accelerates so poorly at highway speeds that the time window to safely complete a passing maneuver usually ends by the time I'm only halfway through the pass. It's not a power issue as it has the same horsepower and only ~12% less torque than the Armada. It's not a drive-train issue, as everything is functioning correctly, so I suspected the factory 3.08 axle ratio must be undergeared, and you all confirmed that.

Going to larger tires is really a different issue and a different question:
I have the Premium Smooth Ride suspension with the Nivomat rear shocks/conventional front struts. I'm going to keep the Nivomat rears and replace the front struts with Bilstein 6112s to level the ride and more importantly, to reduce oversteer and body roll (I have a lot of twisty, hilly, mountain roads where I live). As the 6112s are a good bit firmer than the OE front suspension, I thought I could mitigate that "feeling every pebble on the road" firmness with a little bit larger tire for more cushion.

I will keep the OE tire size for now, and sometime down the road, I plan to swap to 3.73 or even 3.91 gears and go up to 33" tires.

 

[vcode]

I appreciate all the feedback so far -you guys have been very helpful and gave me the answer I needed. I didn't want to ask a super lengthy question, but since you're asking, here's the background:

I traded out of a bone-stock Nissan Armada that had approximately the same horsepower and only about 40 lb./ft. more torque than my Tahoe. It would overtake cars at highway speeds like they were standing still. The Tahoe accelerates so poorly at highway speeds that the time window to safely complete a passing maneuver usually ends by the time I'm only halfway through the pass. It's not a power issue as it has the same horsepower and only ~12% less torque than the Armada. It's not a drive-train issue, as everything is functioning correctly, so I suspected the factory 3.08 axle ratio must be undergeared, and you all confirmed that.

Going to larger tires is really a different issue and a different question:
I have the Premium Smooth Ride suspension with the Nivomat rear shocks/conventional front struts. I'm going to keep the Nivomat rears and replace the front struts with Bilstein 6112s to level the ride and more importantly, to reduce oversteer and body roll (I have a lot of twisty, hilly, mountain roads where I live). As the 6112s are a good bit firmer than the OE front suspension, I thought I could mitigate that "feeling every pebble on the road" firmness with a little bit larger tire for more cushion.

I will keep the OE tire size for now, and sometime down the road, I plan to swap to 3.73 or even 3.91 gears and go up to 33" tires.

What year Armada?

 

[SilverSurfer5300]

What year Armada?

'08

 

[vcode]

'08

08 had a 5 speed and 3.36 gears. Motor Trend said the HP rating was way low based on their 0-60 time. Just don't expect any big changes by changing gears. I bet you'd be lucky to see a 1/2sec improvement in 0-60 going to 3.73's......

 

[DaveO9]

I'm looking at a 3.08 to 3.42 or (more likely) 3.73 conversion myself. But I'm keeping stock size tires. For the front, I'd probably just get a junkyard carrier and swap it completely. 3.73 units are available from the 07 and 08 4L60 trucks. But for the rear I will probably change ring and pinion, as I want to swap to a 9.5 semi-float 14 bolt at the same time, and those weren't available in 3.73.

My question to the group: How feasible is it for the DIY'er to change out ring and pinion? I've always heard it's a no-no due to the shimming challenges to get just the right pattern. But I've watched some vids and it seems possibly more doable than I thought. You can get a slightly oversized bearing for the pinion that makes a trial and error approach (by using paint and looking at the mesh pattern) easier. But worst case, I have a coworker that used to run a family transmission shop, and maybe her husband (one of the mechanics) would be able to help me get it set right.