Another Gear Ratio Thread...

by **Cable810** » Mon Jan 14, 2013 11:17 am

I'm planning on upgrading to an 8.6" rear axle locked with 3.73s. From all the reading I've done if changing ratios in the rear you must match the front to be able to use 4wd. Just my questions is will the 3.73s be too much for a 31.6" tire? I plan on going to a 32.2 when my Grabbers are up. I read and read and I didn't really find what I was looking for in answering that question. From what reading I have done it sounds like it may be a tad to much for the 31.6" but ok for the 32". Also from other reading I've done if running 33s you have to re-gear or risk tranny problems. With up grading to the 8.6 axle I have to make sure the axle is between the years 02 and 04? I would need the Front Axle Assembly from an EXT OR the Differental Assembly? I'm assuming the Assembly.

Correct me if I'm wrong on anything.

by **bartonmd** » Mon Jan 14, 2013 11:25 am

You do know that some of these came stock with 4.10 final drive with the stock tires, right? ETA: 3.73 is a middle of the road ratio with even stock sized tires.

Mike

by **Cable810** » Mon Jan 14, 2013 11:28 am

No I didn't know that.... Learned something new today! So in a sense I'd be totally fine with 3.73s and 32s.

by **bartonmd** » Mon Jan 14, 2013 11:37 am

Yeah, 3.73 are acceptable up to maybe 33"? I guess if I was re-gearing anyway, I'd go 4.10 with anything larger than stock, with the I6, but if that's the deal you've got, then that's cool.

Mike

by **boog2006** » Mon Jan 14, 2013 12:12 pm

bartonmd wrote:I guess if I was re-gearing anyway, I'd go 4.10 with anything larger than stock, with the I6

Mike

Im only planning on this once. Really looking into this myself and I'm torn between 4.10 or 4.56. I seem to remember Fishsticks commenting that his towed like a boss even with 35's...that's what I want

by **navigator** » Mon Jan 14, 2013 12:38 pm

unless you have a deal on the 3.73 rear axle,I would try to hold out for the 4.10s. In my area on car-part.com they seem to be a little harder to find and more expensive than the 3.73s.

I also think you can use any front diff from 02+ SWB or LWB.

One additional thought, I would expect that 3.73 with say 33s on the interstate would get a little better gas mileage than 4.10s, does anyone have any real world experience with that? The reason I mention it is if you spend a lot of time on the interstate the 3.73 could make sense.

by **bartonmd** » Mon Jan 14, 2013 12:53 pm

navigator wrote:unless you have a deal on the 3.73 rear axle,I would try to hold out for the 4.10s. In my area on car-part.com they seem to be a little harder to find and more expensive than the 3.73s.

I also think you can use any front diff from 02+ SWB or LWB.

One additional thought, I would expect that 3.73 with say 33s on the interstate would get a little better gas mileage than 4.10s, does anyone have any real world experience with that? The reason I mention it is if you spend a lot of time on the interstate the 3.73 could make sense.

Depends on the setup, I guess. With Kyle's 33's and 3.73 final drive ratio, it would drop lockup all the time, on any kind of grade. Even with 33" tires, if I had the choice, I'd do 4.10.

Mike

by **The Roadie** » Mon Jan 14, 2013 12:56 pm

I have 3.73 and 33's. I only use the interstate to get to the trails. Mileage is irrelevant compared to trail performance. Yes, I wish I had 4.10 or 4.56, but as I've posted a hundred times, I have yet to be thwarted by a trail due to lack of gearing in 4LO. They always deny me due to lack of ground clearance or traction. Airing down approximately doubles the traction, and I've gotten up some 40+ degree sandstone fins in Moab. So it's clearly not worth it to me to regear, although the trails I seek out now are ways to get to scenic destinations, and not the destination in themselves.

For a 2002, any year front diff or rear axle will work. Only issue is a old rear axle in a NEWER vehicle with stabilitrak.

by **fishsticks** » Mon Jan 14, 2013 1:56 pm

boog2006 wrote:
bartonmd wrote:I guess if I was re-gearing anyway, I'd go 4.10 with anything larger than stock, with the I6

Mike

Im only planning on this once. Really looking into this myself and I'm torn between 4.10 or 4.56. I seem to remember Fishsticks commenting that his towed like a boss even with 35's...that's what I want

It's no Cummins, but I can tow a full U-haul 6x12 in D3 without trans temps getting too bad. Keep in mind I have a cooler as well. Without a trailer, it generally stays in lockup on the highway even with a full complement of passengers and gear.

My truck is still slow.

4LO gives a very satisfying LURCH when you drop it into gear. :mrgreen:

by **Cable810** » Mon Jan 14, 2013 2:07 pm

Ok makes more sense now. I don't really do a whole lotta Interstate driving. In fact my TB hasn't seen the interstae since I've owned it. I kinda think 4.10s might be overkill for 32s unless y'all think otherwise. Mike you mentioned Kyle had an issue with drop lockup? From reading it has something with the TCC and tranny temprature? I do plan on a cooler.

by **fishsticks** » Mon Jan 14, 2013 2:30 pm

If you have to change the front gears anyway, go 4.10 (or 4.56).

I rocked 4.56s on 32s for months and loved it.

by **bartonmd** » Mon Jan 14, 2013 2:43 pm

Cable810 wrote:... I kinda think 4.10s might be overkill for 32s unless y'all think otherwise. Mike you mentioned Kyle had an issue with drop lockup? From reading it has something with the TCC and tranny temprature? I do plan on a cooler.

Yeah, Kyle and several others' TBs would drop lockup at higher speeds on any kind of hill. Yeah, it's a transmission heat thing, but the I6 TBs have a very loose converter that makes a ton of heat when it's unlocked. You can get rid of that heat with a cooler, but it still makes that heat, and is still hard on the fluid between the converter and the cooler. If it drops lockup on the highway, at normal highway speeds, with normal (small) rises in terrain (I wouldn't even call them hills), then there isn't enough power at cruising RPM. You can do 2 things to fix that. You can do a bunch of engine mods to get enough power, or you can gear it so that you're up farther into the power that the engine already makes, at cruise speed. IMO, if you're trying to go as cheap as you can, right now, sure, do the 3.73 gov-lock 8.6 and find a 3.73 front diff. If you want to have the best setup for whatever you want to do, and only do it once, go 4.10 or 4.56 now.

IMO, if you're already switching, Kyle did it right. Find an SS setup and swap in the 4.10 front end, get an open carrier for the 9.5" rear end, and put a lunchbox locker in it... Or leave the LSD in the rear for now, and eventually put an open carrier and lunchbox in it.

ETA: Car-Part says that a couple places in norther Indiana have SS rear ends for $440-$550 with 35-60k on them. Search near you.

Mike

by **HARDTRAILZ** » Mon Jan 14, 2013 2:52 pm

3.73 is not a worthwhile upgrade period. I have no clue what you have been reading or how you interpretted some things how you did. If you are going to regear go 4.56 even with 30 inch tires. If you are going to swap a stock setup in for gearing, go 4.10. We literally cannot gear our trucks too low because the gearing is not available.

by **fishsticks** » Mon Jan 14, 2013 3:00 pm

Hey Kyle, have you ever checked clearance from the top of your diff to the floorpan? I wonder if there's any chance of clearance issues for someone without a body lift.

by **HARDTRAILZ** » Mon Jan 14, 2013 3:06 pm

No, but an SS clears it without a body lift...

by **fishsticks** » Mon Jan 14, 2013 3:13 pm

HARDTRAILZ wrote:No, but an SS clears it without a body lift...

True, just wondered since a lot of us push the rear end back via control arms, and the SS doesn't see nearly the suspension flex we do.

by **bartonmd** » Mon Jan 14, 2013 3:14 pm

HARDTRAILZ wrote:3.73 is not a worthwhile upgrade period. I have no clue what you have been reading or how you interpretted some things how you did. If you are going to regear go 4.56 even with 30 inch tires. If you are going to swap a stock setup in for gearing, go 4.10. We literally cannot gear our trucks too low because the gearing is not available.

This is especially true given that the Atlas engine loves to spin.

Mike

by **Cable810** » Mon Jan 14, 2013 3:27 pm

Thanks for all the helpful info. Looks like I will be searching for a 8.6" with 4.10 and a locker. Thanks again guys!!

by **Moots1288** » Mon Jan 14, 2013 3:33 pm

This is why NYC sucks I've been searching for a while and can't find a goddamn thing. Atleast at a decent prce

by **Cable810** » Mon Jan 14, 2013 3:38 pm

Last few questions. I can get an 04 rear axle? Also for the front diff would it be the Front Axle Assembly OR the Differental Assembly? I'm leaning on the Diff Assembly.

A quick search on Car-Parts rendered me this

2002
Rear Axle
GMC Truck Envoy XL: A4WD,GT5,G80,158K,DISC 157,811 $400

Offroad Trailblazers and Envoys