12 bolt rear gears

[andyh1956]

If you can find a set of 2.56 from a car you can have the pinion stem ground down on a crankshaft grinder or even a lathe w/ a grinding attachment to the same diam as a truck pinion. The cars had beefier pinions & bearings than the trucks (Go Figure?)
However be advised that SOME truck housings are too small up front & those high speed pinions just won't fit in there.
The faster the ratio the physically larger the pinions got!

[64jmb]

Originally Posted by LostMy65

Many will say that 3.42 is a good balance.
But for me, my tires are only 26" tall, so I'm going with 3.08. - which I already have in my barn. 4 years later, and I still haven't got it installed.
Currently have 3.73, with a TH350, and it winds up too high.

Ya I have the TH350 as well winds up way too much. I think that the 308's is probably the what to go.

[AcampoDave]

Originally Posted by 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.

My 4 speed is not an overdrive. The 2.76 seems to make 4th feel like overdrive but it's still 1 to 1. Having had both 3.08s and 2.76s I would say 3.08 is better for everything except flat open roads. If you don't plan on stop and go city driving, hills, pulling a light weight boat or waverunner, going camping, doing an occasional burnout, etc then the 2.76 is fantantastic. I personally do enjoy doing all the activities I just listed so i'm switching to a 3.08. The extra 2 or 3 hundred r.p.m at hwy speeds will be worth the trade.

[64jmb]

Ya, I knew that it is was 4 speed.. i just figured the 4 gear seems like overdrive and bogs down Ill bet... I want to be able to cruse at 75 to 80 on the freeway and not be at anything more than 2K so it will make my truck run hot and be very hard on the motor. I think a 2.76 is the tallest you can put in a 12 bolt I am assuming. Do you know where I can get a set a 12-bolt 2.76's?

[LostMy65]

There's a cool excel spreadsheet floating around here that you can plug in the ratio for each gear in your transmission and after putting in your tire size and rear gear ratio, it will spit out the rpm at practically any speed at any gear.
So for me, 50% of my drive to work is 40mph.
So a rear gear that serves someone best at 60mph may totally lug me at 40mph.
I've done a lot of searching on this topic, and from my understanding, your engine's best rpm is when it produces its most vacuum while cruising.
If that's the case, get a vacuum gauge, cruise down the highway and note what rpm your engine is producing the most vacuum. Then use that noted rpm with your wheel size to calculate the best gear for your most common day to day driving speed.

[64jmb]

I actually have a factory VAC gauge on my truck and it works great. The spread sheet is the ticket, I have that and been looking at it. All based at 55 miles per hr and tires size. I am pretty sure I have 4:10's based on my rpm at 55 is about a good guess is 2600 to 2700 which according to that chart puts me about a 4:10.

[The Rocknrod]

Originally Posted by LostMy65

Many will say that 3.42 is a good balance.
But for me, my tires are only 26" tall, so I'm going with 3.08. - which I already have in my barn. 4 years later, and I still haven't got it installed.
Currently have 3.73, with a TH350, and it winds up too high.

I put 3.42 in my rear end but I'm going to use a 200R4. Now I think that I should have used a 3.72. Which means more that just changing ring & pinion on these 12 bolts. Maybe it'll be okay, the 200R4 starts out at 4.11:1 ratio in first so maybe... But then there is the top end, I-carumba

[LostMy65]

I've shy'd away from the 200R4 and 700R4 because the posts talking about how critical setting the TV cable is.
Fortunately I have two full rearends - a 3.08 & 3.73. So I can go either direction if I need to.

[64jmb]

ya I have the turbo 350 and not going to charge that it is a great tranny. I am thinking 308's will be sufficient I am sure. If anyone has the 2:76 would be great, let me know.

[61K10]

the biggest problem with the 200r & 700 is people installing them with no knowledge. the tv cable is not a kick-down like on a 3.50.
Put the gears in that you want. 2.76-3.08 not much difference.

[64jmb]

ya I will be next year. 3.08's most likely as I know no one that has the 2:76's... If any one has a set of the 2:76's please let me know! Thanks

[C10matt]

I'd go with 3.08s there is only about a 250 rpm difference between the two. th350 trans only has a 2.52 1st gear so combining that with a 28.8 tire and 2.76 gears it'll be kind of a dog from a stop.

[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.

[Clyde65]

12 bolts are 63-82, then they we 10 bolts.

[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.

[LostMy65]

Here's the excel spreadsheet I was talking about.
It's a zip file because the forums won't let you attach an xls.
So after you download it, you have to unzip it.
It's really cool because you can set up the ratios for different transmissions, plug in your wheel size and rear gears, and it will tell you what your rpms will be at for different speeds at each gear shift.
http://67-72chevytrucks.com/vboard/s...76#post4288276