Looking for some suggestions on rear end/suspension, etc

[DodgeGuy]

So we've owned our '74 Challenger for a year now and couldn't be happier that we decided to get back into the classic Mopars that I had when I was a teenager and we were first married.

In any event, after a summer season of driving, I've decided that I definitely want to replace the Sure Grip 3.91 gears, with something more "highway friendly".  I was pretty sure I wanted to do this when we first bought it a year ago, but thought I'd give myself a year of driving it to be sure, and I'm sure.


My thought was to go with a 3.23 Sure Grip.  I'm thinking switching to 3.55 isn't worth the time, but 3.23 would be the "sweet spot" of still having plenty of take off kick, but also provide for a comfortable cruise at speeds of 60+ MPH, where the engine isn't working quite as "hard".

I have the 8 3/4 rear end.  So what am I going to need to make sure of as far as rear-end compatibility?  I know there is a number stamped on the housing assembly.  Also, any suggestions as to where to get one?

Also, while I'm replacing the rear-end, is there anything else I might want to replace (other than seals/fluids)? 


In addition to replacing the gearing, I noticed back at the beginning of summer that my car sits a bit "crooked" when you look at it from back.  The driver's side seems to sit about 1/2-3/4" lower than the passenger side. 

What might be causing that?  As far as the rear suspension goes, I'm thinking things are pretty much stock, but once I'm done winterizing my garage (new entry doors and garage door seals, plus an install of a ventless, propane heater) I'm going to get her up in the air on jack stands and will start looking seriously at it.

What kinds of things could I do to improve the rear-suspension?  I'm looking for any and every suggestion people can offer, just so I know what the possibilities are.

Thanks for any and all help!

[Brads70]

I'd give some thought to keeping the 3:91 and going to an OD transmission 

[1 Wild R/T]

I'd give some thought to keeping the 3:91 and going to an OD transmission 

[DodgeGuy]

I'd give some thought to keeping the 3:91 and going to an OD transmission 

It's a factory 4-speed.  Is there a 5-speed manual?  If there is, I had no idea such a thing existed, lol

Secondly, I think I would like to stick with the stock manual transmission to be honest.

[Chryco Psycho]

Nothing special about the swap , you can order a new center section from Cass @ DR Diff , any center section will fit , axle splines are all the same , they are a simple diff , replace the inner axle seals while it is apart , they only real hang up is the U Joint size 7260 or 7290 so ideally you want to match the yolk up to the U joint but there is also a conversion joint available ,
I have done numerous swaps using the Mopar A833 alum OD from trucks , they are cheap & easy to find & will bolt in , you need to haver the front brg retainer turned down to 4.80 " & have a belhousing with the 4.80 opening  to make it work , the factory shifter will work but you have to work with eh the 3-4 shift rod & levers , other than that it is a direct bolt in  just another idea for OD

[RUNCHARGER]

A better swap is the Passon OD 4 speed. It will bolt in and has better ratios and is a lot stronger than the original Chrysler OD 4 speed. Also Passon makes an OD 5 speed that bolts in however it is around $5k I believe. Honestly the Passon OD at $2k is not that much more than changing your rear end ratio by the time the smoke clears.

[GY3R/T]

    Overdrive with a 3.09 first and 3.91s.... Give me a break !
    a set of 3.55s  OR Passion 5-Speed.

[Brads70]

    Overdrive with a 3.09 first and 3.91s.... Give me a break !
    a set of 3.55s  OR Passion 5-Speed.

I have 3.06 first gear ratio and 4:10 ( 4L60) ? Works great for me? 

[GY3R/T]

    Overdrive with a 3.09 first and 3.91s.... Give me a break !
    a set of 3.55s  OR Passion 5-Speed.

I have 3.06 first gear ratio and 4:10 ( 4L60) ? Works great for me? 

Works great for what ??  Pulling stumps ???

[Chryco Psycho]

I ran a 3.09 1st with 4:30 gears for 9 years depends what yoou want to do , 1.4 60' or triple the speed limit .
3.91 with 3.09 OD gives you 2.77 final drive  on the highway , seems about right to me

[DodgeGuy]

Thanks for the replies so far!  I love the idea of the OD 5-speed transmission, but I'm not popping for an almost $6,000 dollar transmission.


I did a search for the new rear-differential through Dr. Diff, and $1,400 dollars is much more palatable.  I would prefer a good, used one, maybe even one I could trade my 3.91 for, but I'm not opposed to buying a new one at that price.



Any suggestions on the slight sag to the driver's side (leafs?) and any potential upgrades in that area?

[Brads70]

Any suggestions on the slight sag to the driver's side (leafs?) and any potential upgrades in that area?

How is the front end for being equal side to side. I ask as when you adjust the torsion bars it also effects the rear heights  some. How old are your leafs?

[DodgeGuy]

A better swap is the Passon OD 4 speed. It will bolt in and has better ratios and is a lot stronger than the original Chrysler OD 4 speed. Also Passon makes an OD 5 speed that bolts in however it is around $5k I believe. Honestly the Passon OD at $2k is not that much more than changing your rear end ratio by the time the smoke clears.

So after doing some research I understand what you meant about the Pass OD kit.  This is a kit that allows me to rebuild my current factory manual transmission into a 4 speed OD transmission?

[Chryco Psycho]

Any suggestions on the slight sag to the driver's side (leafs?) and any potential upgrades in that area?

How is the front end for being equal side to side. I ask as when you adjust the torsion bars it also effects the rear heights  some. How old are your leafs?

I meant to comment on this the leafs are soft by comparison so if one side sits low it is the front torsion bar that is the problem it works in an X pattern

[RUNCHARGER]

Yes: Check your torsion bar adjustments first to make sure they are even, that will effect the rear as well.
Yes: The Passon OD4 kit can be installed in your present transmission and is a great way to keep your current 3.91's but give you an overdrive top gear to get the best of both worlds.