3.40 12 bolt gear ratio?

[gramps77]

I just found my build sheet under the seat and it states 3.40 gear ratio.
First Ive ever seen a GM 3.40. I was thinking it may of had 3.08s with the th-350 it DID have. Now Im debating installing some 3.73s with the new 700R4. But now 3.40 to 3.73 doesnt seem much of a change. Although when it goes into overdrive it does seem to be right on that bogging line at 65-70 on the freeway

[gchemist]

The gears should be 3.42s. What size is your tire? If it's larger than 28" the 3.73 gear set will work better. If it's small than 28", I would stick with the 3.42's.

[Keith Seymore]

Originally Posted by gramps77

I just found my build sheet under the seat and it states 3.40 gear ratio.
First Ive ever seen a GM 3.40. I was thinking it may of had 3.08s with the th-350 it DID have. Now Im debating installing some 3.73s with the new 700R4. But now 3.40 to 3.73 doesnt seem much of a change. Although when it goes into overdrive it does seem to be right on that bogging line at 65-70 on the freeway

Regarding the original gear ratio - need more detail. What model and year truck, and what RPO does it show for the gear ratio? Also - keep in mind you are assuming it's never been changed during the life of the truck. Just because it was built with a certain ratio doesn't mean that's what is still in there now.

Regarding the overdrive - going from 3.42 (ish) to 3.73 is just about right when going to an overdrive, which will be in the .70 to .73 range, in my opinion.

K

[gramps77]

It was my grandfathers pickup that generally only got regular maintenance and the only changes from stock were the addition of tripod towing mirrors and a new short block at some point. It even still has all the original vacuum lines and is only missing the snorkel from the front of the the radiator support to the air filter housing. So Im sure the gears are the same. It is a 1977 350 4 barrel that had the th-350 and was considered the "heavy half ton".
The RPO states "HE4 3.40 RR axle"
and according to this, its exactly as I stated, http://www.nastyz28.com/gm-rpo-codes/rpo-hk.php

[Keith Seymore]

Looks like you are exactly correct.

Here is the info packet from the GM Heritage Center archives, which shows all of the base and option content for a 1977 Chevy pickup.

http://www.gmheritagecenter.com/gm-h...olet-Truck.pdf

Page 27 shows the powertrain combinations, which show the HE3 3.07 axle ratio as base, and the HE4 3.40 axle available as an option.

K

[68Timber]

I went through this last weekend on my 79, had to pull the cover. Found 3.40 gears and a Gov-Loc. From what I could find, the 3.40 gears are exclusive to 12-bolt truck rear axles.

[gramps77]

Originally Posted by BRUISER

I went through this last weekend on my 79, had to pull the cover. Found 3.40 gears and a Gov-Loc. From what I could find, the 3.40 gears are exclusive to 12-bolt truck rear axles.

Apparently the Gov Lock is also something that isnt very reliable but it hasnt failed yet. Or at least I cant find a receipt for any rear end work that was in the pile.

[74 stepside]

My 73 has a 12 bolt with 3.40 gears also. A little nicer on the freeway since I have a t400 trans.

[Corts60]

I also have 3.40's in my truck. It's a great combination with a t400 and a gear vendors. Final drive with the overdrive in is 2.67.
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[motornut]

i have the 1HE4 -3.40 G-80 as well,in the 79 jimmy

[88Jimmy4x4]

my 1976 also has 3:40, took cover off and counted teef. I think its a good ratio, not to high for in town driving and not too low for hway

[406 Q-ship]

Originally Posted by gramps77

Apparently the Gov Lock is also something that isnt very reliable but it hasnt failed yet. Or at least I cant find a receipt for any rear end work that was in the pile.

It is not the gov-lock that is the issue, when driven right they live forever. Most of the failures are from the offroad guys who put wheels way heavier and when they get the one wheel spinning at more than 20 MPH fast then the stopped wheel. When you get a wheel spinning with a gov-lock you cannot floor it cause when the locker engages it is like dropping the clutch a 5000
RPM....parts will break.

[Firebirdjones]

Originally Posted by 406 Q-ship

It is not the gov-lock that is the issue, when driven right they live forever. Most of the failures are from the offroad guys who put wheels way heavier and when they get the one wheel spinning at more than 20 MPH fast then the stopped wheel. When you get a wheel spinning with a gov-lock you cannot floor it cause when the locker engages it is like dropping the clutch a 5000
RPM....parts will break.

They do have a reputation as being nick named a Gov-bomb I've broken them before just stomping the gas on flat pavement with a bone stock 79 350 that probably made all of 180 hp

Which is why when I aquired a 78 12 bolt differential to swap into my 72 blazer, I pulled the gov-loc and installed a new Eaton pro series posi to rid myself of a potential explosion. They just aren't the hot setup if you have something that's warmed up a bit and you do any spirited driving.

[54Caddy]

My truck has 3:07 with a 26.5 tall tire and a 700r4 is that low ratio killing me should i up up to a set of 3:42s?

[Firebirdjones]

Eh, that's personal preference. You still have a pretty short tire on there. With the 3.06 first gear it will still get going easy enough.

The real issue is whether or not it has trouble keeping the converter locked up in overdrive while cruising down the interstate. If you live in hilly terrain and hit slight grades and find the converter unlocking quite a bit to keep from losing too much speed,,,,then a gear change would help that.

Or you could find ways to pull more torque out of the engine with cam/intake/exhaust mods so it has more grunt to pull it through those situations with light throttle loads, and in turn help to keep that converter locked under light load situations.

[54Caddy]

No lockup in my transmission. Non lockup convertor and no wiring.

[Firebirdjones]

Giving up some efficiency without the lockup, and about another 2-300 rpm.

Since you don't have it, I'd leave the 3.07 gear in there myself. .70 overdrive on it's own really isn't much of a drop from 1:1.

[gchemist]

Originally Posted by 54Caddy

No lockup in my transmission. Non lockup convertor and no wiring.

Install a converter. You have nearly the same setup as I do. Except, 355 HP and 405 ft/lbs of torque available for use. The ZZ4 even at low cruise speeds moves great due to engine torque. A stock engine would benifit from a locked converter. You can install a 1800-2000 stall converter and keep your setup.

[54Caddy]

lockup wasnt added during build due to future engine build i was going to do. But going LS now. i have all 4l60e internals in my 700, 18 clutch pack, raybestos z pack kit. All sorts heavy duty drum.

[Firebirdjones]

You can run any stall you wish and still keep the lockup function. That's the beauty of it, have your cake and eat it too. I run a 3400 stall behind an LS engine with the lockup converter. Kicks butt around town and still locks up for cruising down the road like a stocker would.