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#1 |
18 Till I Die
Join Date: Mar 2005
Location: Alberta, Canada
Posts: 1,522
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Any Noticeable change going 3:42 to 3:07?
Well the old Gov-lock 3:42 Posi packed her in last night. So now I'm on the hunt for a another diff. I plan on getting another diff and re-bearing it and doing a full axle housing swap. The problem is finding a 3:42 non posi is almost impossible around here. I don't really need a posi for the truck, it is just a daily driver/hobby. Its a 1987 Silverado 350 Fuel Injected with a 700r4 trans. I found a 3:07 rear end that is in pretty good shape. Will I have a problem with power? mileage? Did the 350/700 combo come with any other diff ratios other than 3:42? Or should I be looking for something else? Thanks Guys
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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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#2 |
1965 Chevy C10, 2005 4.8L/4l60
![]() Join Date: Aug 2004
Location: DFW Texas
Posts: 8,541
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Re: Any Noticeable change going 3:42 to 3:07?
I would keep the 3.42, the 3.07 change wont get you much MPG if anything, since you have the 700, keep the gears you have.
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Clyde65 Rebuild of Clyde http://67-72chevytrucks.com/vboard/s...84#post8338184 69 Aristocrat Lo Liner build http://67-72chevytrucks.com/vboard/s...84#post7561684 ![]() ![]() ![]() support our troops! |
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#3 |
Registered User
Join Date: Jan 2009
Location: league city, tx
Posts: 180
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Re: Any Noticeable change going 3:42 to 3:07?
well my 1987 has a 2.73 gear. I was told that this was the only year they put that ratio in these trucks and for good reason! I'm looking to get a 3.42 to swap into it. I have lousy off the line acceleration because i don't have enough torque getting to the wheels. I had an 1989 with a 3.07 and it was okay but that rearend broke a couple teeth on me and i did put a 3.42 gear in it and it was much better off the line and towing things so i liked it alot better. gas mileage for me wasn't that much different since i mainly drive it around the city and maybe 30% on the highway and now i do the same thing with my 1987. I would rather have more fun with it than get the best mileage possible out of it. I probably get around 14 city, 17-18 hwy now and maybe 12 city, 15-16 hwy when i did the swap to the 3.42 in my 1989. I also had the holley 502-6 trottle body on it and no other mods. stock other than that. oh and they are both 5.7, 700R4 TBI K ENGINE 2wd long beds that i'm talking about.
OH and what wheels are those? If i'm not mistaken, these trucks have a narrow width between the back wheels than the front wheels. Are your rear wheels backspaced to make them even with the front set?? could you tell me what you have? Last edited by jesdude7789; 12-15-2011 at 11:30 AM. |
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#4 |
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?
There would be a 10% decrease in power going from 3:42 to 3:07 ( i didn't know they made 3:07, i thought it was 3:08?)
i don't know of the exact formula of why/how degree increase/decrease when changing gear ratios, but i have a chart here that lists all the possible combinations, so if anyone is curious i can answer any proposals. For example, going from 2.41 to 4.10 gives a 70% increase in power to the rear wheels. Personally, i wouldn't worry too much about going higher, but that's just me---i don't think i would tow/haul anything too heavy in a 1/2 ton right now coupled with time constraints is why i feel this way if i was in this situation. Also, if you go with the 3.07 and need "help" with the speedo gears, i have info on that as well. |
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#5 |
Registered User
Join Date: Jan 2009
Location: league city, tx
Posts: 180
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Re: Any Noticeable change going 3:42 to 3:07?
you're right simplex. it was a 3.08 It's been a long time since i owned that other truck haha. I would be interested in knowing what your chart says about going from my 2.73 to the 3.42? And i would like the info on the speedo gear too. My speedo isn't exactly accurate as it is.
Thanks! |
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#6 |
Registered User
Join Date: May 2007
Location: East Haven, CT
Posts: 882
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Re: Any Noticeable change going 3:42 to 3:07?
My 87 305/700 combo with 3.08 gear..... wish it had 3.42. Great gas mileage at 18mpg (driving slow), but the truck struggles on hills.
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87 Chevy 2wd SWB: 305/700r4 Dual Exhaust, K&N Filter, Injector Pod Spacer, TBI Spacer, Belltech Front Sway Bar, Corvette Servo, Western Chassis 4/6 and c-notch |
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#7 |
Registered User
Join Date: Jul 2009
Location: washington
Posts: 4,178
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Re: Any Noticeable change going 3:42 to 3:07?
wouldnt go with 3.08 if u can help it sometimes a low gear gives better gas mileage than a high gear ie 2.73 not to mention more fun
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377 sbc thumpr cam autogear m23 muncie 3:73 Detroit trutrac 3''spintech prostreet mufflers xpipe 1 3/4 headers build thread !http://67-72chevytrucks.com/vboard/s...d.php?t=577217 Iroc gauge threadhttp://67-72chevytrucks.com/vboard/s...d.php?t=554511 |
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#8 |
Registered User
Join Date: May 2010
Location: laurel hill Fl
Posts: 645
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Re: Any Noticeable change going 3:42 to 3:07?
High gears aren't very bad at all. Mine has a 2.73 but I've never had a hill problem i can cruise along at 70+ on the interstate with or without trailer I rarely go over 2.5k rpm. I'm getting about 20-23 with my 305/700 combo. Which i couldn't do without the 2.73. for dd it's great. only drawback is added stress on the power train and low gawr.
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1985 C10 Custom Deluxe LWB 305/700r4 [ Goldfinger ] ![]() 1999 k3500 crew cab dually 6.5tdi/4l80e |
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#9 | ||
Senior Member
Join Date: May 2004
Location: Caldwell,Tx.
Posts: 3,648
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Re: Any Noticeable change going 3:42 to 3:07?
Quote:
Quote:
Frank
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Born and proudly residing in a Red State ! |
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#10 | |
Registered User
Join Date: Jul 2009
Location: washington
Posts: 4,178
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Re: Any Noticeable change going 3:42 to 3:07?
Quote:
wish i can afford the 3.73 dodge 8,3/4 rear for it
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377 sbc thumpr cam autogear m23 muncie 3:73 Detroit trutrac 3''spintech prostreet mufflers xpipe 1 3/4 headers build thread !http://67-72chevytrucks.com/vboard/s...d.php?t=577217 Iroc gauge threadhttp://67-72chevytrucks.com/vboard/s...d.php?t=554511 |
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#11 |
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?
OH and what wheels are those? If i'm not mistaken, these trucks have a narrow width between the back wheels than the front wheels. Are your rear wheels backspaced to make them even with the front set?? could you tell me what you have?[/QUOTE]
Those are the old Slots that I found used and polished up. 10's on the back 7.5" on the front Thanks for the input guys. Next question... If I swap in a 12 bolt will the driveshaft bolt up or is the u-joint a differnt size? Might have a line on one ![]()
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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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#12 |
Moderator
![]() Join Date: Oct 2008
Location: Wentworth, NH
Posts: 4,951
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Re: Any Noticeable change going 3:42 to 3:07?
I had a stripped down base model 85 C10 with the SM465 and 2.73:1 rear. The 2.73 gears, 305 V8, and delay wipers were about the only options on her. That combo got me 14mpg hwy so I'm not complaining.
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#13 |
Registered User
Join Date: Jul 2009
Location: washington
Posts: 4,178
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Re: Any Noticeable change going 3:42 to 3:07?
[QUOTE=
Next question... If I swap in a 12 bolt will the driveshaft bolt up or is the u-joint a differnt size? Might have a line on one ![]() just did this on a friends..... no they wont, you need a new u joint pressed in
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377 sbc thumpr cam autogear m23 muncie 3:73 Detroit trutrac 3''spintech prostreet mufflers xpipe 1 3/4 headers build thread !http://67-72chevytrucks.com/vboard/s...d.php?t=577217 Iroc gauge threadhttp://67-72chevytrucks.com/vboard/s...d.php?t=554511 |
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#14 |
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?
Just the U-joint? or the driveshaft too? Whats different about the U-joint?
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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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#15 | ||
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?
Quote:
Quote:
i think riding a 10-speed bicycle alot could be very educational regarding gearing/gear selection. |
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#16 |
Registered User
Join Date: Jul 2009
Location: washington
Posts: 4,178
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Re: Any Noticeable change going 3:42 to 3:07?
the u joint size is different on the 12 bolt i belive. you can have an adapter u joint pressed in though
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377 sbc thumpr cam autogear m23 muncie 3:73 Detroit trutrac 3''spintech prostreet mufflers xpipe 1 3/4 headers build thread !http://67-72chevytrucks.com/vboard/s...d.php?t=577217 Iroc gauge threadhttp://67-72chevytrucks.com/vboard/s...d.php?t=554511 |
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#17 |
Enjoying left field
Join Date: Jun 2008
Location: Dillon Montana
Posts: 900
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Re: Any Noticeable change going 3:42 to 3:07?
Kelsfine, I have 2 3.07 gear sets for a 12 bolt. I'm right below you in the Idaho panhandle.
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#18 |
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?
Thanks guys..The hunt is on..Need to find something close..Hope to find a 3:42 but If I find a 3:07 I have a set of old GM gears here(GM Waranty) to swap in...
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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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#19 |
Registered User
![]() Join Date: Oct 2011
Location: Wildomar, CA
Posts: 466
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Re: Any Noticeable change going 3:42 to 3:07?
My 85 had factory 305/700r4 w/3:08's in the rear. Trans was too "busy" for my liking and almost idled down the freeway which made for a downshift if I even breathed on the gas pedal. Yanked 'em and put in Richmond 3:73's with a Auburn limited slip carrier and am very satisfied. If I recall it was about $600 bucks for the parts and an afternoon in the driveway.
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#20 |
Registered User
Join Date: Jan 2010
Location: Roanoke, Tx.
Posts: 1,561
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Re: Any Noticeable change going 3:42 to 3:07?
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.
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#21 | |
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?
Quote:
I found that the mid 80s GM V8 engines like a RPM band of 2000 to 2500 as the optimum power band. Outside of that, then you pay in fuel mileage.
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Frank |
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#22 |
MadMan
Join Date: Nov 2010
Location: Hammond, La.
Posts: 490
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Re: Any Noticeable change going 3:42 to 3:07?
Buddy of mine still has his 79" Malibu. All it has is a 305 crate motor, quadrajet, stock manifolds, flowmasters, 700r4 and 3.73 gears. He runs 20" wheels and loves it. He can torch the tires at will with that gear ratio.
![]() When I change mine up I'm going with a 4.10 ratio and 24" wheels in the rear ![]()
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81" Shortbed C10 Full Restoration underway, who needs a budget?? LS Pro Touring Build |
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#23 |
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?
My 79 has a factory 2.73 in it.
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#24 | |
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?
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#25 |
Registered User
Join Date: Dec 2008
Location: Jenks, OK
Posts: 314
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Re: Any Noticeable change going 3:42 to 3:07?
Since its not your daily, I would probably wait for a 3.42 to show up. But if you go another route let us know. I have a 3.42 in my 78 now, but I always wondered if I could get a little better mileage with a taller gear.
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Jay 1978 Big 10 Silverado, 355 + 700R4 + 3.42 + factory tach + A/C + dual tanks + working cruise control = awesome highway cruiser. |
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