Changed out rear end on 2005 Tahoe Ls with a 2005 Z71 Tahoe

sliding9

Original poster
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Oct 3, 2019
2
Alabama
Changed out rear end on 2005 Tahoe Ls with a Z71 Tahoe and having issue with transmission. I am thinking it is from the different gears and computer is reading it wrong. I have no idea what the gears are on the new rear end we got it used from Pull a Part out of a 2005 Tahoe Z71. Does anyone know how to go about fixing this issue. Thanks
 

Reprise

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Jul 22, 2015
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If you're wanting to find out the gear ratio, you could either:

- Raise the rear wheels, Drain the gear oil, remove the cover, and count the teeth on both the ring and the pinion. Use a colored grease pencil (at your local h/w store) if you need to make reference marks while turning the wheel to get a full revolution on the ring gear.

Divide the ring tooth total by the pinion tooth total, and you'll have the ratio.


- Or... put a mark / piece of tape on your driveshaft, where you can see it from a rear tire. Place a reference mark on one of the rear tires. Jack up the rear and count how many rotations of the driveshaft it takes to rotate the tire one revolution. This isn't as exact as the first method, but it'll get you close enough, if you know the available ratios for that MY.

Taking a wild-@ss guess, I'll predict you have a 3.73 in the rear, now; the full-size pickups with Z71 of that era had that ratio around then, IIRC.

If you have 4WD, do NOT drive the vehicle with the front driveshaft connected (in any mode) until you verify you have the same gear ratio front and rear. If you have different ones, leave the front shaft disconnected and keep the truck in 2WD until they're both the same ratio again. Your Service Parts Identification label in your glove box should give you the gear ratio that came stock with your Tahoe (it'll have what's called an RPO code, which you can use to cross-reference -- some examples are GT4, GT5, GU6 (and that is NOT a complete list). If you have G80 or G86, that is a limited slip (G86) or locking (G80) differential. The Z71 diff probably has the G80 locker, btw.

As far as altering the ratio, if you have a different one than stock, it can generally be modified with a GM Tech2 diagnostic scanner (or its successor, which the dealer techs use now). Maybe a Snap-On Solus, but I'm not sure on that one. So you could take it to a dealer, if you don't have access to either of those diagnostic platforms; they'll charge you for (at least) one hour of time to change it (even though it will take them less than half of that.)

Do you have a check engine light on in the dash? What codes are present? Barring that, what is happening with the transmission that you suspect there's an issue with it?
 
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Reprise

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Quick search on '05 Tahoe reveals:
- Only two gear ratios available -- 3.42 (GU6) or 4.10 (GT5).
- Z71 is not exclusive to either ratio above
- Tahoe Z71 could be either 2WD or 4WD
- Engine choices were 4.8L (LS) or 5.3L (LT)
- Optional trailering package did not guarantee a specific gear.

Given that info, I would guess that more 4.8s got the 4.10 gear, even though they only were 10hp short of the 5.3 (but about 50 TQ.) But since the axle ratio could apparently be ordered as a stand-alone option, there's no easy way to tell, short of looking at the glovebox label, or using the methods as per my first post.
 

littleblazer

Member
Jul 6, 2014
9,265
I have an easier method for figuring out the rear gear... get the truck up to about 65 mph... use a gps app on your phone to verify. Take the engine rpms and calculate speed based on known factors... so rpms * 60 = rotations per hour
Divide that by overdrive ratio (.70 for 4l60e)
You now have your input speed to the rear end.
Divide that by what rear end ratio you think you have or solve for it later... I'll solve later.
So what you do now is go online and look up your tire size and find how many revolutions per mile are required... do 1 divided by that number, you now have miles per revolution.
Multiply that by your input speed, you will be in MPH but will not have the rear end ratio accounted for.
Divide that number by the GPS speed and you will have your rear end ratio. If you want I can upload a spread sheet where you just punch the numbers in.
 

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