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#1 |
Registered User
Join Date: Feb 2007
Location: los angeles
Posts: 202
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results from a rear end swap
Hey guys,
Finally got my truck back from oklahoma. I had to leave it up there after i made the drive from los angeles in december for a family issue. Truck ran great going cross country. Only thing that sort of bothered me was the rearend. I have read on here that the 700r4 with 3:08s really arent really that efficient as far as the powerband goes. I average 16 mpg on the highway but i felt like the engine was lugging. I had to keep on the gas alot to keep it up to speed. In low mountain ranges i had to dang near keep it open just to get up them. I really think i can squeeze another 1 or 2 mpg if i upgrade to a 3:42 and get a lil more get up on inclines. It just seems like my engine is working harder than it needs to. Anyone have any actual results from switching out their gears. I would like to hear them. I run a stock 350 with edelbrock 1406 carb and edelbrock performer manifold. Hooker headers etc etc |
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#2 |
Formerly- 1972SuperCheyenne
![]() Join Date: Oct 2008
Location: Wake Forest NC
Posts: 5,782
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Re: results from a rear end swap
I have had good results running a 700r4 with 3.73 gears. Great all around combination. I am sure 3.42 gears would be ok too.
__________________
--Josh My Build Thread:http://67-72chevytrucks.com/vboard/s...d.php?t=799218 A good crossthread is better than Loctite any day. Life is not about what you have, but who you have to share it with. |
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#3 |
Registered User
Join Date: Sep 2009
Location: Quitman, Texas
Posts: 52
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Re: results from a rear end swap
I went from 2.73's to 3.73's with the 700r4. Got better mileage, don't remember exactly how much but it was noticeable. Picked up some time at the drag strip too.
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#4 |
Registered User
Join Date: Feb 2007
Location: los angeles
Posts: 202
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Re: results from a rear end swap
Thanks for the fast response fellas.
. I figure i will get better mileage on the highway and on the street it might drop. I can live with that though. |
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#5 |
Registered User
Join Date: Feb 2007
Location: Austin, TX
Posts: 3,930
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Re: results from a rear end swap
Not exactly what you are looking for but somewhat related - When I got my truck it had an open 3.42 rear end and with the stock engine in it, it ran really well. I had a 3.08 posi rear from my old truck so I decided to swap it in. After that the gas mileage went down 2-3 MPG, and it was just sluggish in general. Also ran hotter too.
Eventually I did a cam swap that really boosted the low end torque of the motor and that got back some of the driveability and gas mileage. Just an example that shows how gearing has to be matched to the torque curve of the motor. Honestly, it still has the feeling like it's never in the right gear. Something about being able to do 60 MPH in first gear isn't right either. ![]() A 3.42 gear will work well for you, assuming you are running a 28" tall tire.
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1977 GMC Sierra Grande Last edited by Pyrotechnic; 09-16-2009 at 01:37 PM. |
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#6 |
Registered User
Join Date: Feb 2007
Location: los angeles
Posts: 202
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Re: results from a rear end swap
its pretty close to 28 inches. I have a pretty nice combo as far as my cam and manifold go. I really think the gears will bring everything together. I will proboably stick in an MSD box i have laying out in my garage as well. I suppose that wont hurt either.
Last edited by shrtbx; 09-16-2009 at 01:57 PM. |
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#7 |
Senior Member
![]() Join Date: Jul 2009
Location: Hayden, Al.
Posts: 4,148
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Re: results from a rear end swap
I went from a 2.73 to a 3.42 with a 700r4 and my truck is a whole different animal. It's more responsive and the mileage went up 2-3 mpg on the interstate, but is about the same in town.
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#8 |
Registered User
Join Date: Feb 2007
Location: los angeles
Posts: 202
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Re: results from a rear end swap
what kind of mileage did you average before the swap NLPED.
Last edited by shrtbx; 09-16-2009 at 02:22 PM. |
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#9 |
Registered User
Join Date: Aug 2006
Location: Florida
Posts: 1,684
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Re: results from a rear end swap
i went from 2.73 to 3.73 w/ a 4l80 and it's a different beast now.
mpg? no idea and don't care, too much fun!
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1986 C10 SWB 6.0/6l90e Miami...Flee it like a native |
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#11 |
Registered User
Join Date: Feb 2007
Location: los angeles
Posts: 202
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Re: results from a rear end swap
yeah thats the number im shootin for...Good stuff guys. thanks
Whats the size of a 1986 rear end. 8.5? Last edited by shrtbx; 09-16-2009 at 06:36 PM. |
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#12 |
certified tard
Join Date: Jun 2009
Location: grand junction, colorado
Posts: 182
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Re: results from a rear end swap
i believe its an 8.5" 10 bolt, totally not sure about splines though.
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85 swb bbc 5% tint 16.356 @ 84.81 8/07/09 4800 Ft., bad timing, carb tune, and traction, fugg it. 16.169 @ 84.29 8/28/09 good timing, no carb tune, some traction... 06 Equinox LT AWD, wife's and family ride 03 Mustang V6, CAI, Duals, Pullies, Programmer, Short Throw, wife's dream car?! 90 Eagle Talon, other project / space taker, GOT head!! 08 Silverado 2500 HD, the bad ass work truck |
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#13 |
Senior Member
![]() Join Date: Jul 2009
Location: Hayden, Al.
Posts: 4,148
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Re: results from a rear end swap
If it is the stock rearend it is an 8.5" 10 bolt w/ 28 spline axles. I don't really know why this is relavent, but that's the answer to the previous post.
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#14 |
Registered User
Join Date: Sep 2008
Location: Bailey, NC
Posts: 2,462
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Re: results from a rear end swap
I think he is trying to get info to buy the correct gear for the differential. I have run 3.08,3.42 and a 3.73 in my truck with the stock 305. I like the 3.42 overall. It has a good take off the line and decent highway with the 700r4.
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Jeramy '84 GMC project truck '85 Chevy 4X4 SOLD '07 Chevy Trailblazer SS 5th Annual SHPHC Car/Truck Show Pics http://67-72chevytrucks.com/vboard/s...d.php?t=543605 UPDATED PICS! CHECK 'EM OUT |
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#15 | |
Badass
Join Date: Jan 2009
Location: lox. fl
Posts: 4,586
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Re: results from a rear end swap
Quote:
wish i has a OD
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Think outside the box? -84c10 (1stcar)→My Build← ![]() -00 Saturn 5spd drop a gear and disappear! 1/4:13:90 ![]() |
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#16 |
Still plays with trucks
Join Date: Jul 2009
Location: Spokane, WA
Posts: 3,556
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Re: results from a rear end swap
Try 5.13's and no overdrive
![]() Even with 32" tall tires I am at 3800 rpm at 70 mph. Overdrive is high on my priorities list but I drive mostly in the city and crawl around in the mud & snow so the gearing is great for what I do most. |
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#17 |
certified tard
Join Date: Jun 2009
Location: grand junction, colorado
Posts: 182
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Re: results from a rear end swap
i didnt want to search and i didnt want to be the a hole who asked. i thought it was a pretty clever way to find out!
__________________
85 swb bbc 5% tint 16.356 @ 84.81 8/07/09 4800 Ft., bad timing, carb tune, and traction, fugg it. 16.169 @ 84.29 8/28/09 good timing, no carb tune, some traction... 06 Equinox LT AWD, wife's and family ride 03 Mustang V6, CAI, Duals, Pullies, Programmer, Short Throw, wife's dream car?! 90 Eagle Talon, other project / space taker, GOT head!! 08 Silverado 2500 HD, the bad ass work truck |
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#18 |
Registered User
Join Date: Feb 2007
Location: los angeles
Posts: 202
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Re: results from a rear end swap
Whatever works best for ya man lol.
![]() Last edited by shrtbx; 09-18-2009 at 03:14 AM. |
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#19 |
Still plays with trucks
Join Date: Jul 2009
Location: Spokane, WA
Posts: 3,556
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Re: results from a rear end swap
The 88-up new body style trucks had 30-spline 8.5" 10-bolts but shared the same ring & pinion as the older cars & trucks with the 28-spline 8.5 so it is a nice upgrade if you are breaking the axles on the 8.5 rear end.
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#20 |
Senior Member
![]() Join Date: Jul 2009
Location: Hayden, Al.
Posts: 4,148
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Re: results from a rear end swap
I think you are right, Jeramy. Just for future reference, the spline count doesn't matter when it comes to the gear selection, just the ring gear size...i.e. 8.5" 10 bolt.
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#21 |
Registered User
![]() Join Date: Sep 2007
Location: Marietta, Ok
Posts: 246
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Re: results from a rear end swap
I will have your answers in a few days!! I have an 87 swb 2wd with a 700r4 and 2.73 10 bolt and am about to swap in a 3.42 geared 12 bolt out of an 85 lwb I parted out some time ago. Just finished installing new seals and checking everything out, now just gotta wait for the rain to slack off. but I will give my results asap
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87 black silverado swb, tbi 350, 700r4, 3.40 gears. 2.5 drop spindles , 2 inches off stock coils, flip kit out back. cragers with radial ta's... ongoing piddle project! 1970 Olds rallye 350, street strip 403/350 3.73 posi. Looks stock/runnin 12's! ![]() 86 K5- SOLD! |
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#22 |
Registered User
Join Date: Dec 2008
Location: Washington, MO
Posts: 643
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Re: results from a rear end swap
Anyone try 4.11's with a 700r4? Think they'd be too steep?
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#23 |
Slugish
![]() Join Date: Jan 2005
Location: Earlysville, Va.
Posts: 1,024
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Re: results from a rear end swap
Iran a 3:73 in a turbo 350 --too steep (for me)
think a 4:11 in a 700r4 would be about the same rpm's at any given speed . 4:11 would be great for a drag truck > not so great to cruise |
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#24 |
Senior Member
Join Date: May 2004
Location: Caldwell,Tx.
Posts: 3,648
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Re: results from a rear end swap
A friend had a 93 stepside with a 700 and 3.42's stock . It ran a 295/50/15 on the rear (this was the 90's) and it ran good . He swapped to 4.11's and it was too much in 1st gear . He went back to the 3.42's and was much happier .
Frank
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Born and proudly residing in a Red State ! |
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#25 |
Registered User
Join Date: Feb 2007
Location: los angeles
Posts: 202
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Re: results from a rear end swap
most guys i know with 4.11s in their cars run them only with manual 5 or 6 speeds. Thats on an everyday driver of coarse.
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