Rear axle gear ratio change question

[usraptor]

1970 Cuda 440/512 Stroker, 4-speed, Dana 4:10 rear, Super Trak Pack.  As one of several winter projects that I'm considering undertaking I've been thinking about changing my OEM 4:10 gears to 3:55 or 3:23s. As much fun as the 4:10s are for push you back in your seat acceleration, they pretty much limit any freeway driving to car shows or other events.  I'll be the first to admit that I'm not very knowledgeable when it comes to knowing what exactly I need to change and how difficult it is or how much expertise is needed.  I'm pretty good working on engines but have shied away from working on rear ends.  I had 3:55 gears in my original '70 Cuda with 4-speed and 383 Magnum engine and they performed well on the freeway.  However, with this engine pushing close to 500 HP or more, should I opt for the 3:23s instead?

Also, what exactly do I need to make the change?  Complete new ring and pinion or just one or the other?  And where is the best place to buy from for dependable gears and posi that won't break the bank?

[kawahonda]

The dreaded rear-end gearing question! I just went through this, and it took months for me to figure it out and land on 3.73s. But I have a small block and a 727, so I need "slightly" aggressive gears. I can't see myself ever taking my Challenger w/ drum brakes on a 85 MPH (+5MPH) on the interstate that often. I geared mine for "fine on the highway", "fun in town" while being "doable if you had to freeway it".

There's 1,000 different opinions. Just wanted to chime in and say that you'd probably be pretty happy with 3.23's, especially with a big block since you don't want to spin them that high. I can't imagine anyone recommending over 3.55 in your case being that you want to enable solid free-way driving.

3.23's without over-drive isn't going to be the "bee's knees" either on the freeway, but a nice compromise without adding an expensive overdrive or going too tall to where driving isn't fun anymore.

A lot of what goes into gear selection is rear tire size. What rear sizes do you run?

I'm a huge fan of this calculator and have spent hours with it:

http://www.angelfire.com/fl/procrastination/rear.html

I suggest you start plugging in some numbers to get an idea of what your cruising RPM is going to be.

EDIT: Doctor Differential is who I went with; mine should arrive tomorrow. Expect to spend $1450 for a new center-chunk that's maintenance free and ready to be bolted up. It's not cheap, and I'm not aware of a cheaper way.

[GoodysGotaCuda]

I've setup dozens of gear sets and would not recommend someone try to do it without experienced supervision. It can be a challenge without some know-how not found in textbooks.

That said, you may consider going to an overdrive at some point. 3.91s and a double-overdrive in my 'Cuda is a wonderful, wonderful thing.

[usraptor]

I'm running 275/60X15 rear tires.  Hate to admit my ignorance here, but is the limited slip/posi part of the gear set or can I use the current limited slip/posi with a new gear set?

Also, who did you end up buying from?

[kawahonda]

Ring and pinion are separate from the limited slip mechanism.

It doesn't make any sense to build it yourself from scratch if you haven't done it before. Agree with Goody. It's complicated and you need knowledge/special tools. If you bought it all separately and took it down locally to get it put together: that doesn't make any financial sense compared to just buying one completed and ready to bolt up.

http://www.doctordiff.com/8-3-4-489-case-third-member.html

With 275/60 tires, you may be fine with 3.55s, but no shorter for the interstate without overdrive. I'm not speaking from experience, just with months and months of performing calculations and giving you arm-chair info. Use that calculator and give it a try.

Example:

With 28" tall tires, 1.00 final drive ratio, 3.55 gears, you'll be pushing 3,000 RPM at 70 MPH.
With 28" tall tires, 1.00 final drive ratio, 3.23 gears, you'll be pushing 3,000 RPM at 77 MPH.

That's why I don't give a damn about the interstate!

AFAIK, you should be able to just change the ring and pinion from your current gear set, but that's for a pro to do locally with the knowledge. Good question.

[RUNCHARGER]

A Dana 60 is challenging to setup. You really should have a case spreader and a depth measurement tool to set them up in addition backlash is adjusted by several episodes of pulling the side bearings off the case and changing the shims to move the case side to side in the housing. I would recommend having an experienced shop do it for a one time change. I usually hit the backlash within the first or second try but it's an ugly job. The aftermarket S60 uses adjusters for sideplay and they are much easier to setup than a factory Dana 60.
Four by Four shops setup Dana 60's all the time as they are popular in offroad applications so finding a good shop to setup a Dana 60 isn't too hard.
4.10 to 3.73 is almost a 10% change and probably not worth it. 3.54 is the highest (lowest numerical) gear available for the Dana 60 and should suit your purposes I think.

[anlauto]

Dr. Diff and someone experienced in rebuilding rear axles.

[Rich G.]

Dana rear is great but unfortunately it's not a simple swap the center section in an hour and be on your way. You're going to have to change the ring and pinion gears and have it set up. Not a simple job especially if you never have done it or have the tools to set pinion depth and backlash. Do it wrong and it will be screaming. 3:23 is a nice gear to cruise all around with. You won't win any races in the 1/4 mile but you can go 65mph no problem. Gear Vendors overdrive is another option but not an  inexpensive choice either.

[Chryco Psycho]

3.23 was available for the dana at some point but I have never located a set , 3.54 is the first choice
OD is the second choice , I would stick with the Dana either way

[usraptor]

Thanks for all the advice and recommendations.  Dr. Diff sounds like the place to buy from.  Will probably take your recommendations and have a shop install and set up.  Overdrive unit would be nice, but it's cost prohibitive at this point.  Thanks for the help!