Rear Axle Ratio (Upgrade 3.21 to 4.56): '08 V6 8.25" RWD | Rack, Pinion, Differential
 

Hey Everyone,

This may be my first post so I'll start off by saying hi to all (however few) active members of this forum. It's nice that this forum is still alive, man have I found it tough to find information and walk-throughs on dakota forums (it was so much easier when I had my VWs)! I'd really like to make a combined post on my experiences with the upgrades that I have done over these past 20 months but once I really start to think about the number of different things I've done...the post would be huge and a big investment! So I guess I'll space things out as I go along. This first post is what I am working on now, I need a little bit of guidance here -

In December 2015 I bought my 2008 Dodge Dakota SXT Extended Cab 3.7L V6 w/ Getrag 238; it is rear-wheel drive with the 8.25" axle set (29-spline) at a 3.21 ratio on an open differential. In June 2016 I bought a set of M14 Mambas with 265/50/R20 tires (OD is 30.50"). My aim is to upgrade the open diff to a Detroit TrueTrac while increasing the gear ratio to the neighbourhood of 4.56.

My understanding is that this c8.25 axle has a spline count of 29. When I search online for ring and pinion gear sets for this axle, pretty much every set that I've come across is for the 27-spline axle. So my question is: is it even possible to upgrade the gears on this truck? If I am swapping to the TrueTrac and doing the gear change at the same time, is there something else that I could also swap out and replace that would help to bring this all together? I am open to other suggestions in this direction as well.


Picture album link here =)

https://dodgeforum.com/g/album/8077357

welcome to the forum, The 97'+ had 29 spline axle shafts. Prior to that the 91-96' had 27 spline axle shafts, when looking up the diff, make sure its compatible with your year...as far as the ring and pinion go, axle spline count has nothing to do with it...

4.56 is pretty deep with 30 inch tires...... if you do ANY highway driving at all, you aren't gonna like it very much.

Hi guys - thank you for taking the time to respond to my questions in this thread.

To share a bit more info, I switch to 245/70/r16's for the winter, they are 29.5s, probably even more of a reason for me to heed your advice. I'm thinking the reason for your comment is because at highway speeds where I would currently be cruising 6th gear going 100km/h @ 2190 RPM, with the 4.56s I would be going the same speed now @ 3110 RPM. Is there any reason in particular why I would not like this? Is it because of the fuel consumption; would I expect to notice a big difference on the occassional long distance trip? What gear ratio would you suggest as being more appropriate?

The Detroit Truetrac (part# 912A553) is listed as compatible with axles matching the following criteria:
1997-2009 Chrysler C-Clip, 8.375" ring gear diameter, 1.21" axle shaft diameter, with a spline count of 29. Should I be concerned that my axle is 8.25" and the one listed above is different?

I found a diagram that has been helpful, I just have a few questions if you wouldn't mind.

https://cimg4.ibsrv.net/gimg/dodgefo...79244fbf0f.jpg

* The part circled in blue, this is the ring and pinion, right? So it is just these two parts that I would be replacing?

* The part circled in red, is this the end of the axle where the 29 splines are? Is this what we are talking about when we say that this 8.25" axle has 29 splines?

* The part circled in purple, would I need to replace any of these components? Or are these part of the Truetrac differential?

* Just out of curiosity, what is this part that I have circled in orange?


Thanks again for the help so far!

Blue - yes
Red - yes
Purple - yes if you are going to a posi or a locker
Orange - is your brake caliper

I'm running 4.56 gears on 30.5" tires. 2011 w/ 4.7, quad cab, 2wd and automatic. Highway economy dropped 1-3 mpg depending on speed and terrain. City mileage went up that much or more. Overall my fuel costs stayed about the same.

Detroit Truetrac (part# 912A553)
just looked this up in summit, and yes, it will work perfect in your 8.25 rear axle .

With 30 in tires why not 4.10 gear that is what I have in my 05 4.7 quadcab great at hiway speeds and quicker off the line

Yeah, I gave it some more thought and 4.10s are probably more appropriate for my driving habits.

Guys here is what I am looking at. These are my prices from an automotive distributor with whom I receive preferred rates.

Eaton Detroit Truetrac 912A553 - my cost $545.00 USD
Motive Gear C8.25-410 - my cost $215.10 USD
Motive Gear R8.25RMK - my cost $69.63 USD

The rep at this company gave me the differential master bearing kit number and confirmed it would work for my truck, despite the website not actually listing my year truck under the product applications section.

Are these all of the parts I would need? If I am ripping everything open are there other components that I should have to replace original factory equipment that may have worn down? C-clips? U-bolts?

I've read on the forums that most suggest having gears done by a professional. I am, by no stretch of the imagination, a professional... but I am pretty resourceful and patient... is it possible that I could do this correctly in the garage?

Thanks to all so far who have been helping, it is much appreciated.

Axle bearings and seals maybe? You are doing that much, so far as I can tell, those outer bearings and seals are all that's left. Might also consider doing the rear brakes?