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#26 |
Parts and more parts
Join Date: Nov 2003
Location: Lebo, Kansas (middle of nowhere
Posts: 6,821
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Re: Any Noticeable change going 3:42 to 3:07?
What are you running for a tranny?
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Frank |
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#27 |
Registered User
Join Date: Nov 2011
Location: Tyler Texas
Posts: 145
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Re: Any Noticeable change going 3:42 to 3:07?
turbo 350
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#28 |
Parts and more parts
Join Date: Nov 2003
Location: Lebo, Kansas (middle of nowhere
Posts: 6,821
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Re: Any Noticeable change going 3:42 to 3:07?
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.
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Frank |
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#29 |
Registered User
Join Date: Nov 2011
Location: Tyler Texas
Posts: 145
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Re: Any Noticeable change going 3:42 to 3:07?
I am rebuilding the engine now. Before it had NO power and got 11mpg on the hwy.
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#30 |
Parts and more parts
Join Date: Nov 2003
Location: Lebo, Kansas (middle of nowhere
Posts: 6,821
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Re: Any Noticeable change going 3:42 to 3:07?
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
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Frank |
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#31 |
18 Till I Die
Join Date: Mar 2005
Location: Alberta, Canada
Posts: 1,522
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Re: Any Noticeable change going 3:42 to 3:07?
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?
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2011 GMC Acadia 1969 GMC BBC LS7 2007 GMC Crew Duramax 1987 Camaro Iroc 350 Tuned Port Injection 42000Km 1987 Chevrolet Silverado 350 TBI 1987 Chevrolet Silverado 350 TBI(yes 2) Take me drunk, I'm too home... ![]() Last edited by kelsfine69; 12-30-2011 at 12:05 PM. |
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#32 |
Parts and more parts
Join Date: Nov 2003
Location: Lebo, Kansas (middle of nowhere
Posts: 6,821
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Re: Any Noticeable change going 3:42 to 3:07?
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.
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Frank |
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#33 |
18 Till I Die
Join Date: Mar 2005
Location: Alberta, Canada
Posts: 1,522
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Re: Any Noticeable change going 3:42 to 3:07?
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...
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2011 GMC Acadia 1969 GMC BBC LS7 2007 GMC Crew Duramax 1987 Camaro Iroc 350 Tuned Port Injection 42000Km 1987 Chevrolet Silverado 350 TBI 1987 Chevrolet Silverado 350 TBI(yes 2) Take me drunk, I'm too home... ![]() |
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#34 |
Special Order
![]() ![]() Join Date: Nov 2004
Location: Mt Airy, MD
Posts: 85,859
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Re: Any Noticeable change going 3:42 to 3:07?
Yes,the 3.08 and 3.42 use the same carrier.
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"BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ |
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#35 |
Registered User
![]() Join Date: Apr 2009
Location: New Freedom Pa.
Posts: 1,335
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Re: Any Noticeable change going 3:42 to 3:07?
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 |
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#36 | |
Registered User
Join Date: May 2009
Location: Motor City
Posts: 9,194
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Re: Any Noticeable change going 3:42 to 3:07?
Quote:
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 ![]()
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Chevrolet Flint Assembly 1979-1986 GM Full Size Truck Engineering 1986 - 2019 Intro from an Old Assembly Guy: http://67-72chevytrucks.com/vboard/s...d.php?t=342926 My Pontiac story: http://forums.maxperformanceinc.com/...d.php?t=560524 Chevelle intro: http://www.superchevy.com/features/s...hevy-chevelle/ |
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#37 |
Registered User
Join Date: May 2009
Location: Motor City
Posts: 9,194
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Re: Any Noticeable change going 3:42 to 3:07?
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
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Chevrolet Flint Assembly 1979-1986 GM Full Size Truck Engineering 1986 - 2019 Intro from an Old Assembly Guy: http://67-72chevytrucks.com/vboard/s...d.php?t=342926 My Pontiac story: http://forums.maxperformanceinc.com/...d.php?t=560524 Chevelle intro: http://www.superchevy.com/features/s...hevy-chevelle/ |
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#38 |
18 Till I Die
Join Date: Mar 2005
Location: Alberta, Canada
Posts: 1,522
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Re: Any Noticeable change going 3:42 to 3:07?
Anyone know the torque specs on the carrier to ring gear bolts off hand?
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2011 GMC Acadia 1969 GMC BBC LS7 2007 GMC Crew Duramax 1987 Camaro Iroc 350 Tuned Port Injection 42000Km 1987 Chevrolet Silverado 350 TBI 1987 Chevrolet Silverado 350 TBI(yes 2) Take me drunk, I'm too home... ![]() |
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#39 |
Registered User
Join Date: Jan 2012
Location: flagler beach,florida
Posts: 176
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Re: Any Noticeable change going 3:42 to 3:07?
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.
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#40 |
18 Till I Die
Join Date: Mar 2005
Location: Alberta, Canada
Posts: 1,522
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Re: Any Noticeable change going 3:42 to 3:07?
OK just tossing around ideas...What posi will work for me? Anything in other vehicals that I can use a Posi out of?
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2011 GMC Acadia 1969 GMC BBC LS7 2007 GMC Crew Duramax 1987 Camaro Iroc 350 Tuned Port Injection 42000Km 1987 Chevrolet Silverado 350 TBI 1987 Chevrolet Silverado 350 TBI(yes 2) Take me drunk, I'm too home... ![]() |
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#41 |
BAD BOW-Silverado XST
Join Date: May 2000
Location: Senior Member from Austin, TX
Posts: 6,431
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Re: Any Noticeable change going 3:42 to 3:07?
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. |
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#42 |
Registered User
Join Date: Dec 2011
Location: Arlington WA
Posts: 1,728
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Re: Any Noticeable change going 3:42 to 3:07?
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
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GMC C15 355 SM465 3.73 80 C10 cruiser build thread 79 short stepside crew cab. big and ugly. build thread |
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#43 |
18 Till I Die
Join Date: Mar 2005
Location: Alberta, Canada
Posts: 1,522
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Re: Any Noticeable change going 3:42 to 3:07?
I got everything together but cant find The torque specs for the carrier bearing bolt caps..Anyone help me out?
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2011 GMC Acadia 1969 GMC BBC LS7 2007 GMC Crew Duramax 1987 Camaro Iroc 350 Tuned Port Injection 42000Km 1987 Chevrolet Silverado 350 TBI 1987 Chevrolet Silverado 350 TBI(yes 2) Take me drunk, I'm too home... ![]() |
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#44 |
Registered User
![]() Join Date: Apr 2009
Location: New Freedom Pa.
Posts: 1,335
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Re: Any Noticeable change going 3:42 to 3:07?
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 |
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#45 |
Senior Member
Join Date: Aug 2002
Location: Valley Center KS
Posts: 3,525
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Re: Any Noticeable change going 3:42 to 3:07?
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
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#46 |
18 Till I Die
Join Date: Mar 2005
Location: Alberta, Canada
Posts: 1,522
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Re: Any Noticeable change going 3:42 to 3:07?
Awesome Thanks..And a pic why not
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2011 GMC Acadia 1969 GMC BBC LS7 2007 GMC Crew Duramax 1987 Camaro Iroc 350 Tuned Port Injection 42000Km 1987 Chevrolet Silverado 350 TBI 1987 Chevrolet Silverado 350 TBI(yes 2) Take me drunk, I'm too home... ![]() |
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#47 |
Account Suspended
Join Date: Dec 2008
Location: indisclosed
Posts: 1,515
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Re: Any Noticeable change going 3:42 to 3:07?
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#48 |
Newbee
Join Date: Jan 2010
Location: Phoenix, Arizona
Posts: 1,406
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Re: Any Noticeable change going 3:42 to 3:07?
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.
Last edited by Dunenutt; 03-16-2012 at 10:22 AM. Reason: can't spell |
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#49 |
18 Till I Die
Join Date: Mar 2005
Location: Alberta, Canada
Posts: 1,522
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Re: Any Noticeable change going 3:42 to 3:07?
Yes the pic is deceiving, The pattern is not bad actually and to get it absolutely perfect compromises in other areas would be worse...
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2011 GMC Acadia 1969 GMC BBC LS7 2007 GMC Crew Duramax 1987 Camaro Iroc 350 Tuned Port Injection 42000Km 1987 Chevrolet Silverado 350 TBI 1987 Chevrolet Silverado 350 TBI(yes 2) Take me drunk, I'm too home... ![]() |
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