12 bolt rear gears

[slimjim66]

Hey guys, is 3.08 the lowest (numerical) set you can buy? I’m looking for more of a 2.80 or something like that. Looking for a good highway gear with a 3 speed trans

[Clyde65]

That’s it for a 12 bolt truck.

What driveline you got and tire size?

[slimjim66]

Originally Posted by Clyde65

That’s it for a 12 bolt truck.

What driveline you got and tire size?

12v Cummins, 727TF, 29” tall meats (265/35r22)

[Captainfab]

There were 2.73's or 2.76's, something like that, available.

[slimjim66]

Originally Posted by Captainfab

There were 2.73's or 2.76's, something like that, available.

Now we’re talking, anyone know where I can source a set?

[maydaymike]

The ring & pinions were the same through the 73-87 trucks, right? My 85 half ton shortbed (305/TH350) came with 2.73 gears.

[Captainfab]

Nope. The truck 12 bolt was used '63-'82 and the 8.5" 10 bolt was used '79-'99.

Originally Posted by maydaymike

The ring & pinions were the same through the 73-87 trucks, right? My 85 half ton shortbed (305/TH350) came with 2.73 gears.

[The Rocknrod]

Quick Performance?

[jocko]

Yukon gear
https://www.yukongear.com/diffdetail...2bBolt%2bTruck

[Captainfab]

I dson't think the aftermarket makes gears that high. I had a set that I sold to the poster of this thread and he ended up pulling them back out because they were too high. You might contact him and see if he happens to still have them.

http://www.67-72chevytrucks.com/vboa...d.php?t=485843

[LostMy65]

Play around with this to help you decide what gears would best:
https://www.randysworldwide.com/calculators/rpm/
This thread might also help:
http://67-72chevytrucks.com/vboard/s...d.php?t=507188

[AcampoDave]

Years ago I had a shop put highway gears in a 12 bolt. They put in a 2.76 and told me it was as tall as I could go with that housing. I still have it today and i run it with a super wide ratio Saginaw 4 speed. It's a highway roller in 4th alright. Probably too much for the engine I'm using these days. (283)

[vince1]

I have 3.07's and IMO it is about all the 250 can handle. It's a good combination town and highway gear.

[LVPhotos]

308 is plenty. Save the money for overdrive. Probly pick up a 200r for cost.

[slimjim66]

Thanks guys I found some.

[64jmb]

ok I am in the same boat. I have a 350/turbo350 and it is all new and it screams on my fact tach! At about 45 to 50 MPH I am about 2700 RPM I think I have 4.11 or 3.73 gears and they are no good for the highway at all! I need the same, 2.76 gears for my 12 bolt help!

Where did you fine a new set I hear I need the ring and piñon, carrier, and gears to change mine out. Anyone help??

[64jmb]

I have been repeatedly told I can only get the 3.08's and it would be enough but I want a taller gear for the highway ... not good for the motor and will make it run hotter as well! Any help would be greatly appreciated!

[LostMy65]

Originally Posted by 64jmb

ok I am in the same boat. I have a 350/turbo350 and it is all new and it screams on my fact tach! At about 45 to 50 MPH I am about 2700 RPM I think I have 4.11 or 3.73 gears and they are no good for the highway at all! I need the same, 2.76 gears for my 12 bolt help!

Where did you fine a new set I hear I need the ring and piñon, carrier, and gears to change mine out. Anyone help??

You can figure out what you have for gears by a method where you jack up one side and rotate the wheel and count the rotations of the driveline. But beings you have a tach, you can figure it out by measuring your wheel height and seeing what rpm you are at when driving 55 by using this chart.

[64jmb]

Ya this is awesome! I kept this chart for sure. I think I have 4:11 or 4:27 gears from what I can tell on my factory tach and tire size what is on this chart.

[AcampoDave]

http://ringandpinionservice.com/ I think these guys did mine

[64jmb]

Wow ok. So they had the 2:76 gears for the 12 bolt in stock?

[AcampoDave]

It's been soooo long ago that it's hard to remember. I think it was them and I just dropped the truck off and picked it up done. The whole rear end got rebuilt and the difference was night and day. Initially it was in a truck with a tourqey 350/350 combo on a shorted. Climbing mountains required downshifting to second which was easily good for 65mph and it was a real roller on flat ground. Honestly though, I believe a 3.08 to be the best for all around driving especially if you ever want to tow a boat or haul stuff around with the A/C blasting. It now rolls under a different truck with a 283 and a Saginaw with the 3.11 first gear. This winter I'm swapping it to a 3.08 and a Muncie M22Z to get a bit more punch.

[LostMy65]

Originally Posted by AcampoDave

It's been soooo long ago that it's hard to remember. I think it was them and I just dropped the truck off and picked it up done. The whole rear end got rebuilt and the difference was night and day. Initially it was in a truck with a tourqey 350/350 combo on a shorted. Climbing mountains required downshifting to second which was easily good for 65mph and it was a real roller on flat ground. Honestly though, I believe a 3.08 to be the best for all around driving especially if you ever want to tow a boat or haul stuff around with the A/C blasting. It now rolls under a different truck with a 283 and a Saginaw with the 3.11 first gear. This winter I'm swapping it to a 3.08 and a Muncie M22Z fo get a bit more punch.

What size tires do you have with the 283 and 3.08?
Do you know roughly what mpg you got with that setup?

[AcampoDave]

27" tire and I'm still using the 2.76. The 3.08s didn't combine well with the gear spread of the Saginaw. First and second were too low. Third and fourth were great. Ultimately, I decided to reinstall the 2.76 and save the 3.08 until I got a more appropriate tranny. I've researched the heck out of the new Muncie m22z and they practically set it up for 3.08s. My odometer is way off so l haven't checked the mileage. It's not too bad though, for what it is. I know for a fact that my buddies 350/350 powered trucks use more gas than I do, but then again, they aren't really geeks about timing and tuning their trucks like I am.

[64jmb]

I have a turbo 350 trans with a new crate 350 motor. So a 2.76 should work right or should I do the 308's? That chart was great! I guess if you have a 4 speed you are in over drive which may be too high of gears? I have a 4 speed in my 66 mustang with a 9 inch rear end 2:78 gears and it was great! Not fast off the line but great on the freeway. I had 205-75-15 tires on it as well. My truck is a 350 with a turbo 350 and based on this chart I think I have 4:10 or 4:27 gears. at 55 mile per hr I am at about 2700 to 2800 rpm. I am running 235-75-15 tires which are about 28.8 inches tall. If you look at the chart, it say I am about a 4:11 to 4:27 gears! What do you all think i should put in this because I do want to be able to drive this 75 miles an hr and not have it get hot and may my engine rev to high like it is now.