Axle ratio to tire size input needed

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swathdiver

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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.
 

vcode

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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

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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

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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.
 
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SilverSurfer5300

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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.
 
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vcode

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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?
 

vcode

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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

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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.
 

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