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2nd Gen Dakota 1997 - 2004 Dakota's

3.92 Rear End With Magnum 3.9

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Old 12-12-2013, 12:37 AM
Josh17 Josh17 is offline
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Default 3.92 Rear End With Magnum 3.9

Well I have been researching for a while now, and I figured that getting a 3.92 Rear end for my 1999 2wd 3.9 Dakota would give me the best bang for the buck. I have Found one that is in my price range. Its a 4X4, 12 bolt, 9.25'' Ring gear, 3.92 ratio. I was just wondering if that would work with my 2wd 3.9. If it doesn't would any of you have any suggestions ?
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Last edited by Josh17; 12-12-2013 at 01:39 AM..
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Old 12-12-2013, 09:13 AM
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The spring perches will be on the wrong side. The 4x4 has the springs on top, the 4x2 on the bottom.

Ratio wise it will work fine.
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Old 12-12-2013, 11:37 AM
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Originally Posted by 00DakDan View Post
The spring perches will be on the wrong side. The 4x4 has the springs on top, the 4x2 on the bottom.

Ratio wise it will work fine.
and it's a 9.25 axle, too. Did the 9.25 and 8.25 use the same spring perch spacing? Not that it matters since they'd need to be cut and re-welded, or use a 4x4 leaf spring and shackles
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Old 12-12-2013, 08:03 PM
Josh17 Josh17 is offline
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Thanks for replying In that case then do you guys know where i could just get the gears and swap them ? I don't want to pay 400 dollars for an axle just for the gears.
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Old 12-12-2013, 08:29 PM
atthewmartin114 atthewmartin114 is offline
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id run with the 4x4 rear end. and do and axle flip and level the truck as well as give your seld one hell of a whole shot. but its your call. also gears are pretty cheap from jeggs. installing them il go to the dealer it shouldnt be 400 or even 300 it should take like 3 maybe 4 hours to do it. if you buy them your self and dont need bearing or anything
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Old 12-12-2013, 09:54 PM
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the truck is my dd, so i cant have a whole lot of down time (maybe on the weekends). What would i need to do an axle flip and to level the truck ?
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Old 12-12-2013, 10:37 PM
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i went to a 4.1 rear gears from 3.55s in my 01 dak 3.9 V6. BUT i only drive in town. my MPG was 8-9, with the 4.1 it went to 11-12 MPG. cause it tricked the trany into thinking it was at a higher speed. and it went in to O.D. at 30 MPH. at 35 MPH the tach is 1400 RPM. and was surprised the it had enough power to pull it. NOTE: in town driving only
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Old 12-12-2013, 11:01 PM
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i went to a 4.1 rear gears from 3.55s in my 01 dak 3.9 V6. BUT i only drive in town. my MPG was 8-9, with the 4.1 it went to 11-12 MPG. cause it tricked the trany into thinking it was at a higher speed. and it went in to O.D. at 30 MPH. at 35 MPH the tach is 1400 RPM. and was surprised the it had enough power to pull it. NOTE: in town driving only
basically same story as me, but you probably dont drive with a lead foot or race on weekends :P
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Old 12-12-2013, 11:36 PM
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no not racing at the track, now days. but i DID have the heaviest foot you ever saw. i have never got as good of MPG as others on the same car / truck. i had a 1965 chrysler 300 L 413, single 4 bbl. not duel like most 300s, 727, 3.23 42000 lb. cast iron ext manifold. totally stock. 1/4 mile 16.08 et / 85 mph
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Old 12-12-2013, 11:51 PM
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the 4.10's really do go well with the 3.9's though. If I didn't do any highway at all or was switching my truck out to dedicated race, I wouldn't think twice about 4.56's. A 4.30 would be the sweet spot, but they don't exist for the chrysler 8.25 or even the 9.25 that I know of. The V6's just don't have the down low torque that the V8's do, and the 4.10's make up for it.

Think of it this way. V6 (3.9L) = 75% of a V8 (5.2L). 3.55 is 78% of 4.56. So kinda/sorta by the numbers, a 4.56'd V6 (3.9L) should produce about the same torque as a 3.55'd V8 (5.2L).

That said, the 42RE behind the V6 and the 44RE behind the 5.2 both have a 2.74:1 first gear (keeping the comparison above valid), but the 46Re behind the 5.9 uses a 2.45:1 first gear.

Without all the words:

3.9 L = 75% of 5.2L
3.55 = 75% of 4.56

3.9 L = 66% of 5.9L
2.74 = 111% of 2.45
net change = 74%

74% is close enough to 75% to say that all 3 platforms are close enough that both the V8's with 3.55's are approximately equal to a V6 with 4.56's.

But the loss of fuel mileage on highway would be more than significant.
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Last edited by magnethead; 12-12-2013 at 11:56 PM..
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Old 12-13-2013, 12:20 AM
Josh17 Josh17 is offline
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I appreciate that tid-bit there, but she's a five speed. I don't want to go as far as 4.56s yet I still have to go to college haha. Also any input about doing the axle flip and leveling kit ? I could also just change the gear and not the axle I just haven't found the gear by itself yet.

Last edited by Josh17; 12-13-2013 at 12:22 AM..
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Old 12-13-2013, 02:03 AM
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Originally Posted by Josh17 View Post
I appreciate that tid-bit there, but she's a five speed. I don't want to go as far as 4.56s yet I still have to go to college haha. Also any input about doing the axle flip and leveling kit ? I could also just change the gear and not the axle I just haven't found the gear by itself yet.
gear setup is a meticulous process to do on your own and risk not doing it right. They usually run $250-$350 depending on brand, then 3-4 install hours on labor. My 4.10's were 1K out the door.

I was just saying that 4.56's will bring a V6 up to the V8 level of torque, but at the grave loss of fuel mileage due to higher RPM. Most of the ram guys go to 4.56's because they have a tall OD and tall tire that makes it work. If I was race-only, I'd go to 56's or maybe even 88's in a heartbeat. What i'm going to do instead is keep the 4.10's and run a 26" slick for a 5.00 effective ratio or 24.5" slick for a 5.18 ratio, and just change tires at the track. I can use a stock 15" wheel from the rimyard/junkyard for $35 a piece and pick from two Mickey Thompson tires- #'s 3015 (24.5 x 8 - 15, $203 each) , or 3052 (26 x 8.5 - 15, $183 each). That will also drop the back end down, helping to get some weight transfer to where there's no static weight at.

But I have to get the axle locked up first. Not looking forward to another $800 bill to put a lunchbox locker in the diff carrier.
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Last edited by magnethead; 12-13-2013 at 02:18 AM..
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Old 12-13-2013, 02:45 AM
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That was very informative. Thank you !
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Old 12-13-2013, 07:54 AM
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That was very informative. Thank you !
yea, the shop that I use charges more, but he used to work at a mercedes dealership and has a whole rap sheet of experience. Now he runs an off-road shop in Dallas. I forget what his labor rate is since we get a *slightly* lower rate on our racing stuff, but it's still 3 digits an hour.

I have nitro gears in mine, a quality but low cost brand. USA standard is another such company.

The 4.10's for the chry 8-1/4 are $190
The 4.10's for the chry 9-1/4 are $260
The lunchbox locker for the chry 8-1/4 is $450
The lunchbox locker for the chry 9-1/4 is $480
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Old 12-13-2013, 07:00 PM
Josh17 Josh17 is offline
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I'm guessing I have the 8-1/4 ?
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Old 12-13-2013, 07:25 PM
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I'm guessing I have the 8-1/4 ?
The easy way to tell, count the cover bolts. If you have 10, it's the 8 1/4.
If you have 12, it's the 9 1/4".
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Old 12-13-2013, 07:54 PM
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Originally Posted by magnethead View Post
Not looking forward to another $800 bill to put a lunchbox locker in the diff carrier.
From what I know about you, you would have no trouble at all installing a lunchbox locker yourself, assuming you currently have an open rear diff. I've done several of them myself (Lock-right, Spartan, and some other piece of crap I can't remember the name of) and I can say that if it takes you more than an hour, you're doing it wrong. You don't even need to remove the carrier from the truck. Just pull the cover and the axles (again, assuming you have a stock open carrier now).

Of the three, I would recommend the Spartan for ease of installation. The Spartan and the Lock-right were both trouble-free once installed, but the Spartan was easier and seemed a little stronger just by visual inspection.
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Old 12-13-2013, 09:41 PM
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Originally Posted by Tom A View Post
From what I know about you, you would have no trouble at all installing a lunchbox locker yourself, assuming you currently have an open rear diff. I've done several of them myself (Lock-right, Spartan, and some other piece of crap I can't remember the name of) and I can say that if it takes you more than an hour, you're doing it wrong. You don't even need to remove the carrier from the truck. Just pull the cover and the axles (again, assuming you have a stock open carrier now).

Of the three, I would recommend the Spartan for ease of installation. The Spartan and the Lock-right were both trouble-free once installed, but the Spartan was easier and seemed a little stronger just by visual inspection.
Since I have the 4.10's already, the ring has to come off for the cross-pin to come out. Otherwise you bet I'd be doing it at home. >.>
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Old 12-13-2013, 09:44 PM
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Ah, bummer. It can still be a DIY job, but that certainly makes for a lot more work.
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