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 ?

 

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

 

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.

 

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

 

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 ?

 

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

 

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

 

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.