Any Noticeable change going 3:42 to 3:07?

[piecesparts]

Originally Posted by Tx79chevy

My 79 has a factory 2.73 in it also

What are you running for a tranny?

[Tx79chevy]

turbo 350

[piecesparts]

With a 350 tranny, the RPMs for your truck at 70 MPH is not all that bad. I do not know your exact tire size, but with a 29" tall tire your RPMs are calculated around 2200. That is not bad. However, that same tire size with a 3.42 gear ratio will put you at 2750 RPMs. You can run at 2700 all day and not have an issue, and you will have a good street acceleration capability.

[Tx79chevy]

I am rebuilding the engine now. Before it had NO power and got 11mpg on the hwy.

[piecesparts]

What are your plans for the engine? A simple cam change and if you want to get more, a change of the heads to something like a set of Dart heads is worth some serious HP changes. I put on a set of SR Torquer heads on my 383 and it breathes very well.

http://www.worldcastings.com/product...ron-heads.html

[kelsfine69]

Found a 12 bolt from a 1982 1/2 ton with a 6.2 Diesel and a 700R4, Anyone know what the most comon Gear ratio is in those? Do they even make a 3:42 in a 12 bolt diff?

[piecesparts]

Originally Posted by kelsfine69

Found a 12 bolt from a 1982 1/2 ton with a 6.2 Diesel and a 700R4, Anyone know what the most comon Gear ratio is in those? Do they even make a 3:42 in a 12 bolt diff?

My 84 had a 3.42 geared 12 bolt in it, when I purchased it. It now has a 3.73 gear, we upgraded to the new gearing when Iput the stroker engine in the truck. The 3.42 gearing in a 12 bolt was common.

[kelsfine69]

Another Disappointment tonight...Long story short ended up with a 3:07 diff with a good non posi carrier...

Next question is...do 3:42 & 3:07 gears use the same carrier??

If so I gonna rebuild my diff with the donor Non-Posi carrier and GM 3:42 Gears I acquired from my Buddy...

[special-K]

Yes,the 3.08 and 3.42 use the same carrier.

[Desert1957]

I will tell you the 87 in my SIG had 2.73's when I bought it. Noticed the typical lazy performance off idle everyone here has mentioned. After installing a set of 31" tires , truck was a Dog.

Overhauled the rear with 3.42's and a limited slip Posi, felt like another 200 horsepower under the hood. Gas mileage remained the same.

Personally I would never go back.

Desert

[Keith Seymore]

Originally Posted by grs

How can I find out what my gear radio is? I have a 305 with 700R4, and tbi. I get about 24 on the highway. It is a 1987.

2.73 base, 3.08 optional.

Check your SPID label; if there is no axle ratio shown then the truck came with the base axle. If it shows the RPO GU4 then that confirms it was built with the 3.08.

Whether or not it has ever been changed out is anybody's guess.

K

[Keith Seymore]

All -

In general, my default is always a 3.42. Nice, all around ratio, so I start there and move up or down based on the desired outcome.

For use with an overdrive trans, I go 3.73:1 and have always been pleased with the balance of fuel economy and off the line performance.

K

[kelsfine69]

Anyone know the torque specs on the carrier to ring gear bolts off hand?

[slinginrods]

I have a 3:08 in mine I hate it.mileage sucks and it's a pig.I have run a 3:73 with 29 in tires and 700 r4before .I like it better than the 3:42 but you will sacrifice some mileage but the truck will feel much better off the line.btw the higher numerical gear just transfers torque better like a long breaker bar does.it does not give you more power.just makes the engine work less hard to move the truck through torque advantage,not a torque increase.I believe the ring gear bolts re 85 lbs-ft.
Posted via Mobile Device

[kelsfine69]

OK just tossing around ideas...What posi will work for me? Anything in other vehicals that I can use a Posi out of?

[gchemist]

You are running small tires. The 3.08's will be fine. I run 3.08s on my ZZ4 700R4 combo with 255/60R15 tires. MPG = 18.5 at 70 MPH. ~13-15 MPG if I keep my foot out of it.

3.42 is still a better gear.

Don't forget the 12 bolts. I ditched the stock 83 10 bolt for a 77 12 bolt 3.08. Swapped right in.

[cory d]

im running 350, th350, 2.73 gears and 255/70/15's. i dont think its the stock gearing, i checked the vin and it originally had a 250 adn 3 speed collum shift. It does great on the freeway but the gears are way too tall for driving around town. my engine is old and tired though, should be better after a rebuild and some upgraded parts

[kelsfine69]

I got everything together but cant find The torque specs for the carrier bearing bolt caps..Anyone help me out?

[Desert1957]

Specifications



10-5/8"
10 HOLES
Irregular Gasket Pattern

RG Diameter 8.50"
10 7/16" X 20 LH Threaded Bolts
PINION Diameter 1.626", 30 Spline
CASE: 2.56 & Down, 2.73 Numerically Higher

8620 Material Heat-Treated

Assembly Specifications:

Cover Bolt Ft. Lbs. 20
Pinion Bearing Preload In. Lbs 24-25
Pinion Bearing Preload In. Lbs 10-15 Used
Ring Gear Backlash .005-.009
Ring Gear Bolt to case Ft. Lbs. 90
Side Bearing Caps Ft. Lbs. 60
Desert

[Ackattack]

I put had a 72 with 3.08s in it. I put in a 700R4 and it was a dog on the highway. I swapped to 3.73 gears and it was MUCH better and I got ~2 mpg better. I think 3.42s would be the best gear for mileage with a ~28" tire

[kelsfine69]

Awesome Thanks..And a pic why not

[C-10 simplex]

Here is the chart btw:

http://www.wallaceracing.com/reargear.htm

[Dunenutt]

Originally Posted by kelsfine69

Awesome Thanks..And a pic why not

Hard to tell from the pic, but you may want to re-check your setup. Looks like the pattern is way in on the heel of the ring gear.

[kelsfine69]

Originally Posted by Dunenutt

Hard to tell from the pic, but you may want to re-check your setup. Looks like the pattern is way in on the heel of the ring gear.

Yes the pic is deceiving, The pattern is not bad actually and to get it absolutely perfect compromises in other areas would be worse...