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#1 |
Registered User
Join Date: Jun 2012
Location: ft worth tx
Posts: 135
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Thoughts on 3.08 rear w/ lsx
Anyone run a 3.08 rear with a 5.3/4l60e swap. My 11 gmc has 3.42 in a heavier ride. Looking to make a driver with mpg in mind. Thanx
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#2 |
Hittin E-Z Street on Mud Tires
![]() Join Date: Nov 2004
Location: Greenville, SC
Posts: 23,090
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Re: Thoughts on 3.08 rear w/ lsx
You'll want a steeper gear. Something in the 3.40s would be good but a good ole 3.73 is a great combo with od in these trucks. I have a 3.73 and 700r. The 700r has a .7 od gear. I think the 4l603 has a .67.
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#3 |
Registered User
Join Date: Jan 2011
Location: Chattanooga, TN
Posts: 2,151
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Re: Thoughts on 3.08 rear w/ lsx
I wouldn't go 3.7. I have an '82 K20 with 700R4 and 3.42 and it is a pretty good combo to me. With a Corvette L98 TPI engine, I get 18ish in town and 22ish on the road.
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#4 |
Registered User
Join Date: Oct 2005
Location: Overland Park, Ks.
Posts: 5,216
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Re: Thoughts on 3.08 rear w/ lsx
I have a 70 with 350 & 700R4 & 3:08 rear end. It drives great. If I am pulling a load I can drop it down to direct drive & itr is still not over reved.
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#5 |
Registered User
Join Date: Sep 2012
Location: Nevada
Posts: 40
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Re: Thoughts on 3.08 rear w/ lsx
I'm running the same combo as Wrenchbender, although it likes to bog a little bit I really like being able to go 70 without feeling like the engine is spinning up too much.
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#6 |
Registered User
Join Date: Mar 2005
Location: 10-Uh-See
Posts: 5,609
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Re: Thoughts on 3.08 rear w/ lsx
As long as the engine is stock, it will be fine. If you run a cam and stall, you will definetly need lower gears. 3.42-3.73 gearing is optimal with that combo.
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#7 |
Registered User
Join Date: Jan 2013
Location: Eastern TN
Posts: 1,921
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Re: Thoughts on 3.08 rear w/ lsx
I bet the 3.42s get better MPG thn a 3.07 rear gear. The 3.07 would have the motor lugging more and the 3.42 is not a high rever once you add OD. I never felt that the LS motors made good low end torque but shined in the mid to upper RPMs. Great for muscle cars but terrible truck motors.
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#8 |
Registered User
Join Date: Sep 2011
Location: McAlester Olahoma
Posts: 114
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Re: Thoughts on 3.08 rear w/ lsx
wrenchbender do you have any idea what you get on fuel milage with your combo.
With two bad knees my standard trans is getting painfull. |
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#9 |
Registered User
Join Date: Aug 2012
Location: Neskowin OR
Posts: 1,153
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Re: Thoughts on 3.08 rear w/ lsx
I had an '02 Silverado with a 5.3 and 4L65E, and a 3.42 rear, and I used to get 21-22mpg on the freeway on long trips even going over mountains between here and California. With my current 4.8/4L65E and 3.73 I get 18-19. I'm not sure a 3.07 is going to be much better than the 3.42 honestly, but if you don't tow you'll probably be fine. I'm kinda on the fence with that too, I have a 3.73 posi in my truck and I'm considering dropping to a 3.42 or 3.07... probably 3.42 because I will tow. Good question! it always depends on what you're going to do with it.
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#10 |
Registered User
Join Date: Jun 2012
Location: ft worth tx
Posts: 135
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Re: Thoughts on 3.08 rear w/ lsx
Thanx for the input guys, I've read a lot about 3.08 vs 373 with OD. Trying to get a feel for the 5.3. I love the 5.3 in my 11 gmc which is heavier than the 70 swb I'm putting togather. I haven't seen 342 in the old rears and don't want to rebuild cuz I already have one of each[3.08 and 3.73]I'll never pull anything with this and my ol 72 350 auto w/ ac never bogged even when pulling. But its always great to here from everyones experience. What did we do before the net was invented?
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#11 |
Registered User
Join Date: Jan 2013
Location: Eastern TN
Posts: 1,921
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Re: Thoughts on 3.08 rear w/ lsx
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#12 |
Registered User
Join Date: May 2007
Location: Mooresville NC/Gainesville GA
Posts: 499
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Re: Thoughts on 3.08 rear w/ lsx
The question no body asked is what size of rear tire? That makes a huge difference in gear ratio. I think MPG between 3:08 and 3:73 would only be 1-2mpg but I think the truck will drive better with 3:73.
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#13 |
Registered User
Join Date: Oct 2005
Location: Overland Park, Ks.
Posts: 5,216
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Re: Thoughts on 3.08 rear w/ lsx
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#14 | |
Happy to be here
![]() Join Date: Mar 2000
Location: Cypress, TX
Posts: 39,023
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Re: Thoughts on 3.08 rear w/ lsx
Quote:
For reference, a late model truck with a 305/40/22 (31.50") will see a 1 MPG decrease from each gear set. Going from 3.08, to 3.42 will drop 1mpg, and down to 3.73 will loose yet another MPG. The opposite effect is the lower the gear, the more torque you gain. This equates to more SPG (smiles per gallon) until you pull up to the pump.
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#15 |
Registered User
Join Date: Jan 2013
Location: Eastern TN
Posts: 1,921
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Re: Thoughts on 3.08 rear w/ lsx
A quick glance at his avatar says stock tire size range. Pretty sure he was sticking 38's under a 2wd either. That is why I didn't ask. Either way I would want to see no less than 2k RPM at cruise. No lugging on hills and enough RPM to keep all the accessories happy.
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#16 |
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Join Date: Sep 2004
Location: Lafayette Georgia
Posts: 5,074
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Re: Thoughts on 3.08 rear w/ lsx
I literally just walked back in the door from my first trip with my 5.3/4L60e. I have a 3.08 gear and a 27.8" tall tire. I drove 120 miles through the foothills/mountains of north georgia as well as through friday evening traffic of a couple small towns (stop and go). I shoot for keeping the speed no more then 10mph over the posted limit... But the first third of the trip I kept catching myself running 70+mph in 55 zones... Did better as I got used to the throttle and the "sound". So most of the trip was ran between 55-65 mph. Pulled 20.8 mpg.
I still haven't driven this setup enough yet to have all the bugs worked out just yet. Electric fan temp is still set up for the old first gen small block... probably needs raised a bit. CEL came on when I had the o2 senors repositioned last week... it's still on... not sure what's going on there yet... but. maybe my mpg will increase... maybe it'll get worse. Who knows. |
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#17 |
WTB this ad
Join Date: Sep 2004
Location: Lafayette Georgia
Posts: 5,074
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Re: Thoughts on 3.08 rear w/ lsx
just a btw... according to the calculator on Randys ring and pinion, I should be turning about 1860(ish)rpm at 60mph.
which I've slept since I looked that up... ![]() |
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#18 |
Senior Member
![]() Join Date: Jun 2010
Location: Illinois
Posts: 2,957
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Re: Thoughts on 3.08 rear w/ lsx
I was kind of wondering what kind of RPM's everyone was pulling with their gear and tires sizes at around 55-65 mph.
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#19 | |
Registered User
Join Date: Jun 2012
Location: ft worth tx
Posts: 135
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Re: Thoughts on 3.08 rear w/ lsx
Quote:
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