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2006 chevy trailblazer_ls
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Discussion Starter · #1 ·
Hey again everybody.

I've searched around, and have decided I want to add a G80 to my truck via an EXT rear axle swap.

I have the factory 3.42 gears, and while I'm looking for a rear axle, I wanted to know if I should just shell out the extra cash for a front diff as well and get the gears changed to 4.10 or if I'll be fine offroad-wise with just swapping out the rear and keeping the 3.42 gears.

I have noticed when searching junkyards that there are a lot fewer 3.73 and 4.10 gears than 3.42s. For more info, I'm hoping to run 32 or 33" m/t or a/t tires with the necessary lifts and spacers, and I spend a good deal of time on the freeway. I'd really rather not have to spend money if I don't need to.

Thanks in advance for the info,
Phil:tiphat
 

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2007 chevy trailblazer_ls
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Depends on what you plan to use the truck for. Is this your daily driver, and if so, how far/how much driving do you do and does the mileage matter? Going to a bigger ratio will make the mileage less, as well as going to a larger tire you will want to go to the bigger gear ratio which will kill your fuel mileage.:m2:
 

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Discussion Starter · #3 ·
I'm using this as a daily driver, but plan to build it as an expedition vehicle as well (I'm a "multipurpose tool" kind of guy). I'm already aware I'll be losing mpg with tires and all that and in effect lowering my gear ratio. My question is whether I'll be fine with 3.42 gears or if it's worth the extra $500 or so to switch front diffs and get the different rear axle.
 

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2004 gmc
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I did this sort of stuff in Moab.



45-50 degree climbs in 4LO. With 32" tires and 3.73 gears. And never needed WOT, which is risky to all the drivetrain parts.

As I've posted before, I do some insane trails for trailvoys, and have never been thwarted by lack of torque. Traction and ground clearance, yes. But never due to torque or wishing I had a different gear ratio.

AlekG, at the other end of the spectrum, with 4.56 and an electric locker in the rear, can walk up trails that I might need more throttle on, and descend with engine braking only (the other advantage of a good crawl ratio) where I might need the brakes. More than 30 degrees, let's say.

I'd say it depends entirely on your planned expeditions, what terrain you're going to be in, and if you're going alone or with a buddy who can give you a strap assist once a day, let's say. If you're only going to need a strap once a year or so if you don't get better gears, you can PAY some of the gas for somebody better built to go with you who can keep you going.

If you were east coast I'd say don't bother. If you have aspirations for some of the Sierra and Moab trails I've seen, you might keep looking for good junkyard prices. IME, they KNOW the 4.10s are rare, and the front diff alone can cost as much as $750. You'll make a bit back selling the old parts, but I bet it's a $1000 total upgrade rather than $500.

Still a very cheap way to get a G80 already installed with the bigger ring gear. :D
 

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2007 gmc envoy_sle
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If money were no option (within reason), I'd have 4.10s if going bigger than 32"...unless there was a 5.3L under the hood.
 

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2005 chevy trailblazer_ls
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with 33s probably

I have the feeling with 33s and only the 3.42 gears he will have some problems going up hills without a decent speed going into it. I could be wrong but i think it would be a good idea to get the bigger ratio.:m2:
 

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2005 chevy trailblazer_lt
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I've got the 3.42s and 32" tires. Again, I haven't had any issues when dealing with torque... especially after getting the PCM tuned. The trans used to downshift for every hill, but not anymore.

Tuning the PCM for some extra grunt was my personal alternative to changing gear ratios, but your specific plans may warrant the ratio change. As Bill said... it kinda depends on your trail goals.
 

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Discussion Starter · #8 ·
Thanks everyone... since my goal is for a vehicle similar to James's, and he says he doesn't have any issues with his, I'm gonna go for just the 3.42 EXT rear end... Although the 3.73 and 4.10 rears aren't more expensive from what I've found (~$350 w/G80), I'm not keen on replacing the front either. So, new rear and PCM4Less tune here I go! At least when I get the money... I like to plan ahead :)

And I didn't know you had 3.42s James... glad to know we can still wheel with cruising gears lol

-Phil
 

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Thanks everyone... since my goal is for a vehicle similar to James's, and he says he doesn't have any issues with his, I'm gonna go for just the 3.42 EXT rear end... Although the 3.73 and 4.10 rears aren't more expensive from what I've found (~$350 w/G80), I'm not keen on replacing the front either. So, new rear and PCM4Less tune here I go! At least when I get the money... I like to plan ahead :)

And I didn't know you had 3.42s James... glad to know we can still wheel with cruising gears lol

-Phil
If you change the rear end you will have to change the front end as well.

when i had 3.73 gears and oem stock tires the gas mileage is ok. But, when we added 22 in rims and heavy brake caliper the gas milegage drop tremendously along with the requirement of more power throttle.

But, when we added the 4.10, got rid of the front diff, and installed and upgrade tc, the gas mileage inproved a little along with more power on performance. (lost a few pounds)

yet it is true that the higer the gear ratio the better the acceleration but the lower the max speed and vice versa.

If your looking for something for DD i would go with a little higher gear ratio to help it move the truck with real ease on the throttle. There is no point of losing max speed, unless your considered racing on track.
 

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Discussion Starter · #11 ·
Sorry, I should have mentioned... I'm planning on swapping the current SWB rear axle with 3.42 gears and open diff for the LWB rear with 3.42 gears and the G80. This should do well enough for my purposes after I get the PCM4less tune.
 

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Sorry, I should have mentioned... I'm planning on swapping the current SWB rear axle with 3.42 gears and open diff for the LWB rear with 3.42 gears and the G80. This should do well enough for my purposes after I get the PCM4less tune.
unless your thinking of keeping the stock wheels then it should help the gas mileage.
 

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Discussion Starter · #13 ·
*scratches head* whaddya mean? I would assume my gas mileage would only be affected slightly by using the heavier axle with the locker... the gearing is the same as what I already have, and the mileage is gonna drop anyways when i put the 265/75/16 tires on there after adding spacers.
 

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2002 chevy trailblazer_lt
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It all comes down to your final crawl ratio (4lo). The more technical, rugged the trail, the higher you want your crawl ratio so you can ease over obstacles nice 'n slowly and in a controlled manner:

First Gear: TC: Axle Ratio: Total:
3.05 x 2.7 x 3.42 = 28.2 (Your TB)
3.05 x 2.7 x 3.73 = 30.7 (Roadie & me)
3.05 x 2.7 x 4.10 = 33.8 (If you upgrade to 4.10)
3.05 x 2.7 x 4.56 = 37.6 (AlekG :cool::excited:)


To give you some perspective on the '09 Jeeps:
4.46 x 4.0 x 4.10 = 73.1 (Rubicon :drool )
4.46 x 2.72 x 3.21 = 38.9 (Sahara/X )



As Roadie always says... It all comes down to your mission of trails...
 

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2003 chevy trailblazer_ls
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I would just save time and money and get your existing gear set in the replacement differential. Then it will give you time to save for a new rear and front if the situation were to come up in the future. Worst case scenerio you sell your replacment differential for what you put out for it. The only thing you lose is the time to install it if you do it yourself. Who knows they may perform better than you are expecting and not require replacement.
 

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It all comes down to your final crawl ratio (4lo).
And for the benefit of folks finding that nice chart in future years, any auto transmission has an implied crawl ratio benefit factor of between 1.5 and 2.0 to 1, just because of the torque converter. So when you're comparing an automatic to a manual transmission for rock crawling, we're in the vicinity of 60-to-1.

Note the tire size doesn't factor into this particular figure of merit. It's always assumed that if you're rock crawling, you're already at 31-33" minimum tire size.

And just to point out why we don't do WOT rock crawling, the I6 is rated for maybe 275 ft-lbs of torque. Minus some driveline efficiency factor (30%?). So let's say 200 ft lbs can get to the driveline. Multiply that times a 60-to-1 crawl ratio through the driveline, and that's 12,000 ft-lbs available at the axle shafts coming out of the differential. If you don't have all four tires on the rocks with equal traction, you could send all that torque through ONE rear tire. For a few milliseconds.

A typical Dana 44 rear axle strength - 4600-5000 (tested by Warn Ind.)

Our rear axle is not as stong as a Dana 44, even.

So this supports what I've found on the trails. We are not going to be denied by lack of torque. What gets us is ground clearance and lack of armor over the sensitive bits. And traction, depending on the trail material, steepness, tire pressure and tread design, and moisture content. ;)
 

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And for the benefit of folks finding that nice chart in future years, any auto transmission has an implied crawl ratio benefit factor of between 1.5 and 2.0 to 1, just because of the torque converter.
Interesting. Wouldn't all rock crawlers use an auto tranny if that were the case? Does the torque converter still give the same ratio in 4lo, or does anything else change in that respect (edit: w/ gear selector in 1st)

On another note, reviewing efi live stats yesterday it was interesting to see the 'transfer case ratio' dynamically changing while driving in 2hi. Factoring in the torque converter ratio as well, maybe?
 

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Wouldn't all rock crawlers use an auto tranny if that were the case?
They don't (in competition), and it must be for robustness, especially at extreme angles. They can also use dual transfer cases to get an extra extra LO, and their normal TC ratios are 4 or 6 to 1.
Does the torque converter still give the same ratio in 4lo, or does anything else change in that respect (edit: w/ gear selector in 1st)
The torque converter factor is constant, and applies in all ranges and gears.
On another note, reviewing efi live stats yesterday it was interesting to see the 'transfer case ratio' dynamically changing while driving in 2hi. Factoring in the torque converter ratio as well, maybe?
I have no idea. I thought it was just a display of a static register that's set to 2.7 to 1. But the software is perfectly capable of being buggy and showing the wrong value. ;)
 

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2007 chevy trailblazer_ls
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Yep, any half decent torque converter will nearly double your torque at certain rpm/load ranges...

The reason they don't use them, other than the fluid thing, is control... or at least perceived control, and engine braking... All 4 locked in, going down a hill in first, is a little more consistant than being on the brakes, I guess..

Mike
 
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