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Upped the gears

1K views 24 replies 14 participants last post by  ggravant 
#1 ·
So tonight i removed my 3.21 gears and open diff and installed 3.92s and trac-loc. It took me about 5 hrs. and i am happy as hell on the performance change. I lost the third gear bog and am sure to lose a few tenths of a second in the 1/4 mile. I am going to go to the track in a week or two and see exactly what it does. I have had a personal best of 16.0 with not many prior passes in my life. I have high hopes for mid 15s i feel it has it in it. I guess time will tell.
 
#3 ·
I didn't know 3rd gens. came with 3.21's.
 
#5 ·
I have the 6 speed manual trans so i guess that is why it had the 3.21 it is a slt so it wasn't cause it was a base line truck either.
 
#14 ·
I've got the same truck only an 08. I am definitely going
to do a gear swap as the 3:21 is way too tall. I find you
really have to slip the clutch in normal driving and reverse
is about useless. Why didn't you go with 4:10 ratio and where
did you get your parts from? Good post BTW!

Thanks..Ed
 
#6 ·
ya they were the bottem end gear available only on the 6-speeds. 3.55s were optional.
 
#25 ·
It is all about the math ...

manual tranny with 3.21s:
3.21 * 0.79 (6th gear ratio) = 2.54 crank revs per axle rev

auto tranny with 3.55s
3.55 * 0.67 (5th gear ratio) = 2.38 crank revs per axle rev

So his truck with 3.21 is effectivly a lower ratio than the typical autos with 3.55s

Also ...
3.92 * .79 = 3.10 CRs/AR

4.56 * .67 = 3.06 CRs/AR

So now with his 3.92s he has a lower ratio than an auto with 4.56s :D

I was REALLY wanting to buy an 09 RC/SB with the new 4.7L, 3.92s and a manual tranny, but NO SOUP (TRUCK) FOR ME!
 
#9 ·
I've got an 08 4.7 with the auto and it came with the 3.21 rear end. With the 310 hp engine its not as bad as you might think. However, I've got a set of 3.92s waiting for install, but I've been undecided about maybe going with the 4.10 instead. If 3.92s is a good enough change then I think I'll go with it.

Thanks for posting your experience.
 
#10 ·
What are the rpms' at highway speed with the 3.21's??? What is your mpg on the highway??
 
#11 ·
I see you did the install yourself. How was that?
 
#12 ·
Highway with 3.21s was around 2000 at around 75mph and when i went from Vermont to New Jersey i actually got 20 mpg. With the 3.92 i am guessing about 2400 to 2500 at 75 mph.
For the install i am an auto tech at a dodge dealer and have done those diffs. plenty of times. Being that our special tool room is completely un-organized it took me about five hours from pulling the truck in, to correcting the speedometer in the computer and going for a test drive.
 
#17 · (Edited)
The engineers probably chose the 3.21s because of epa regulations and trying the get the mpgs higher for highway IMO.
 
#18 ·
Nice man. You'll definately gain a few 10-tenths in the 1/4. For comparison; I had 3.92's w/open diff in my previous 4x4 Ram and went to 4.56's w/DTT and I gained about 3/10's in the 1/4.
 
#21 ·
like previously posted, i work at a dodge dealer and have built these diffs. multiple times and double checked all specs. like backlash and turning torque.
 
#20 ·
how is it driving now? seeat of the pants test i bet went awesome:D
 
#22 ·
Yeah, she moves now. I am glad that i finally did this.
 
#24 ·
no mine is the older style 4.7. In 07.5 they released the 310hp 4.7 if i am not mistaken. I don't have official documentation on what will fit and what won't I just heard from one of the guys in the shop that can hold all of the weird information in his head told me he heard of some fitting issues. I didn't want to chance it and also when i went through the starscan (chrysler's pcm scan tool) the only options that i was given for gear set sizes were 3.21, 3.55, and 3.92. If i went to 4.10 i wouldn't have been able correct the speedometer without an aftermarket scan tool.
 
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