Is an 8 1/4", 2.45:1 diff housing exclusive?

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Duggie

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Just noticed a leaking rear axle seal on my 8 1/4", 2.45:1 rear end. I've enjoyed looking at rear ends my whole life, but never gave them more than superficial guy type thoughts.
:popcorn:

This axel leak got me wondering if this is the (winter) time to swap out the 2.45 gears and do the Jeep rear disk swap. I read an 8 1/4" write up on an early Cuda forum (http://www.earlycuda.org/techdocs/Jeep8.25Swap.pdf) that, "Units with a 2.45:1 gear ratio have a special differential case that will not accept any other gear ratio." Say it 'aint so!
:wtf:

And as long as I'm asking for advise, what rear end gears are people happy with when it's a /6, 4spd. Hopefully this little Dart is on its way to being a daily driver with mild performance mods at the most. Our maiden voyage down and up the west coast shoreline had me downshifting to 3rd every time I met a hill doing 60-65 mph or less. Steep incline starts are a bit of a drag too.

Thanks in advance for the info!:thankyou:
 
Just noticed a leaking rear axle seal on my 8 1/4", 2.45:1 rear end. I've enjoyed looking at rear ends my whole life, but never gave them more than superficial guy type thoughts.
:popcorn:

This axel leak got me wondering if this is the (winter) time to swap out the 2.45 gears and do the Jeep rear disk swap. I read an 8 1/4" write up on an early Cuda forum (http://www.earlycuda.org/techdocs/Jeep8.25Swap.pdf) that, "Units with a 2.45:1 gear ratio have a special differential case that will not accept any other gear ratio." Say it 'aint so!
:wtf:

And as long as I'm asking for advise, what rear end gears are people happy with when it's a /6, 4spd. Hopefully this little Dart is on its way to being a daily driver with mild performance mods at the most. Our maiden voyage down and up the west coast shoreline had me downshifting to 3rd every time I met a hill doing 60-65 mph or less. Steep incline starts are a bit of a drag too.

Thanks in advance for the info!:thankyou:

Awh crap. I just realized I put this in the wrong forum. Geeze oh flip! :BangHead:
 
Do a 8.8 ford rear , 3.73 and you can get disc.

I've read a bit about the 8.8 Ford swap, but don't know that I need the extra beef. I doubt I would exceed the design limits of an 8 1/4 with a mildly tuned /6.
 
My BOG friend had a 170 with a (unknown to us at time) 2.45 rear out of a Cordoba. That car couldn't climb a steep driveway! We gave him a 225 and then we found out the rear was an ultra short 360 sled rearend. never liked 2.76 gears try to find some 2.93s or one step better, 3.0x?
Carrier break: Chrysler 7.25" 2.47 & Down 2.76 & Up...ouch!
 
Get an overdrive, and some ~3.55:1 in any rear you pick.
 
The quick answer you need is yes, a 2.45 will not take another ratio.
the pinion is in a different spot in relation to the ring gear.
ask me how I know...
 
"Units with a 2.45:1 gear ratio have a special differential case that will not accept any other gear ratio." Say it 'aint so!
To answer the question, yes, the 2.45 CASE is a one ratio deal. But AFAIK, the CASE here talked about, is the thing that that ring-gear is bolted to. So then, you should be able to swap in a different case with corresponding different gears. But that gets expensive, when you gotta hire a guy to do it for you, cuz it's gonna require a new set-up; rear gears do not just swap right in.
EDIT;
@motorpirate says it is in fact a no-go, so planB it has to be.

Plan B
The better idea is to swap a complete rear end under there with gears already installed.

And finally, what ratio you install will depend heavily on if the 4-speed that you currently have, is or is not, an overdrive unit, which it sounds to me, that it is. I say that because with a regular 4-speed,and 2.45 rear gears 65mph would be ~2800rpm in 3rd/2020 in fourth, whereas, with an overdrive, third is direct so 65=2020rpm in third/1480 in overdrive.
So no, you cannot climb anything at 1480rpm. I'd be surprised to hear that a slanty could even maintain 65 at 1480, no-lol.
Furthermore
your starter gear with the regular 4-speed and that 2.45 is, 6.52 and that is sure to be very lazy. But the overdrive is nearly as lazy at 7.57, just about 16% better.

So again, which rear gear you choose, will very much depend on which transmission you already have, and how much hiway rpm that you are willing to put up with. And how much more take-off performance you want. You can't really have it all with any of the Mopar boxes, but you can bias them to fit your needs, just a lil.
Here are the ratios;
2.66-1.92-1.40-1.00, the standard box
3.09-1.67-1.00-.73, the od box
3.09-1.92-1.40-1.00, the Commando box.

For a hot-rod get-away, you need a starter gear of about 10/1. To cruise at 65=2400 you need a 2.94 final drive ratio.
The only box that can ballpark that is the od box. As already recommended, the 3.73 Jeep rear will get you a starter of 11.53, and a cruiser of 2.72 so 65=2250, both of which are excellent.
The fly in the ointment is that you only have 3 gears to get you to 65, and they are kindof far apart. So, then, your slanty is gonna need some ballz to get there.
Whereas
with the standard box,and 3.73s, take off is still pretty good with a starter of 9.92, but 65 will be about 3080. You can climb just about anything at 3080, and the six is still pretty smooth up there, but whether or not you want to travel far at that rpm, is a personal matter.

A happy medium is 3.23s and a Commando box for 65=2670, and a starter of 9.98, and now you have four gears to get you to 65mph. It's my favorite combo.

All calculations were done with an 83" rollout which is a tire size of about 26.4 tall. I'm guessing ~205/75-14

Hope that helps
 
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The quick answer you need is yes, a 2.45 will not take another ratio.
the pinion is in a different spot in relation to the ring gear.
ask me how I know...

Finally after all the BULLSHIT somebody answered your question. Thanks, @motorpirate for being straight and to the point like no one else could.

To further the point, yes you do have other gear choices on the 2.45 carrier, but they are all numerically lower gearsets.......and there ARE a few. lol I think they go down to like 2.11 or some ridiculousness. But that's goin "the wrong way". LOL To upgrade with a numerically higher gearset, you will need another style carrier.
 
To answer the question, yes, the 2.45 CASE is a one ratio deal.

No, it's not. The 2.45 case will accept other gearsets....as long as they are numerically lower....and there ARE some out there.
 
It looks like 2.24:1 was the lowest numerical gearset available. @greymouser7 would want to put that behind an OD like a dummy.
 
As I understand it

2.25 & 2.45 gears for the smaller 8 1/4, -several here on FABO have said that the 1997 + newer 8 1/4 axles will hold these gears or the later axle gears will fit in these cases. One way (or the other) is what I have heard. But with the lesser size inside this econo 8 1/4, it’s probably 2.71 or small gears that fit.


2.71, 2.94, 3.21, 3.55, 3.73, 3.91 for the 60’s through 1996 8 1/4 axles

older axles are 27 spline
Newer axles are 29 spline
When Ordering parts It is important to know which you have.
Previously I had a hard time distinguishing/finding older, 27 spline sure-grip rebuild kits vice 29 spline kits.

As for operating a double overdrive (a833 O/D + 2.45)
1. 1976 Feather Dusters/Dart Lites had 225 /6 engines With this combination.
2. GM does it regularly with a T-56 utilizing a .5 overdrive-typically in front of 3.73 gears.
Combined 1.865 GM final gear ratio vice .73 X 2.45 = 1.7885 final ratio.
People driving vettes with a t-56 have squeezed over 25mpg with the .5 X 3.73 gears on the highway.
I was hoping to negate the steep 2.45 gears with a tall 3.49 gear from an NV3500 like a b body guy on the sister site did.

the point Rusty is moot as I am going through divorce, raising/paying for my kid, and closing on a house in a few months-my Mopar progress, aka budget is nearly standstill.
After taxes and school money next year, I’ll be good. And the divorce by the grace of GOD is actually divine. She doesn’t want my retirement, I get to see my kid, and now she is
Confessing to me that she doesn’t know where her pay checks are going (dozens of small junk purchases). I am better off.
 
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As I understand it

2.25 & 2.45 gears for the smaller 8 1/4, -several here on FABO have said that the 1997 + newer 8 1/4 axles will hold these gears or the later axle gears will fit in these cases. One way (or the other) is what I have heard. But with the lesser size inside this econo 8 1/4, it’s probably 2.71 or small gears that fit.


2.71, 2.94, 3.21, 3.55, 3.73, 3.91 for the 60’s through 1996 8 1/4 axles

older axles are 27 spline
Newer axles are 29 spline
When Ordering parts It is important to know which you have.
Previously I had a hard time distinguishing/finding older, 27 spline sure-grip rebuild kits vice 29 spline kits.

As for operating a double overdrive (a833 O/D + 2.45)
GM does it regularly with a T-56 utilizing a .5 overdrive-typically in front of 3.73 gears.
Combined 1.865 GM final gear ratio vice .73 X 2.45 = 1.7885 final ratio.
People driving vettes with a t-56 have squeezed over 25mpg with the .5 X 3.73 gears on the highway.
I was hoping to negate the steep 2.45 gears with a tall 3.49 gear from an NV3500 like a b body guy on the sister site did.

the point Rusty is moot as I am going through divorce, raising/paying for my kid, and closing on a house in a few months-my Mopar progress, aka budget is nearly standstill.
After taxes and school money next year, I’ll be good. And the divorce by the grace of GOD is actually divine. She doesn’t want my retirement, I get to see my kid, and now she is
Confessing to me that she doesn’t know where her pay checks are going (dozens of small junk purchases). I am better off.

Glad to hear from you.
 
Thanks for the math, AJ. It put us on the moon! My 4sp is not an OD. During my transmission overhaul, the NOS cluster gear I put in was standard, '64 vintage and at 65 MPH my /6 is purring at about 2K RPM, according to my vintage dwell/tach sitting on the passenger seat.


I love this stuff! Thanks!! I found this tire size comparison and played with it like a kid with a new toy. :D

From the recommended 15 page thread at the beginning of this thread, I've have learned the offset 2.45, 8 1/4" rear end is "special". Mom said I was special so I'd think I could relate better to this issue... I did note a comment from Prostocker273 (sorry, I haven't figured out how to reference members with the "@Name" yet) from back in 2016 stating 1995 and older Jeep 3.07 and 3.55 would fit with a carrier swap.

I'm pretty sure I could scratch build a rear end if I can access any needed special tools. I'm pretty good at following a cook book and am reasonably good at monkey see - monkey do.:p

I have to remind myself that all this started with just learning a Cheep Jherokee rear disk swap is a $350 date, and finding a leaking rear axel seal. I had a leaking kitchen faucet once that turned in to a full kitchen remodel.
:mad:
 
at 65 MPH my /6 is purring at about 2K RPM, according to my vintage dwell/tach sitting on the passenger seat.
That would point to a slightly taller tire, with a rollout of 84 inches or 26.76 tall,
like ~215/75-14, or maybe 225/70-14

If you have a 64 cluster in there, then you have a Commando box, my favorite.
And that would mean that 65 =2780 rpm. If you can climb Ok at 2780, that would point to a 3.40 gear, rounds down to 3.23s or up to 3.55s
65mph with 3.23s and 84's is ~2640rpm, and the starter is a respectable 9.98, up from 7.57, or plus~32%
Do you need 3.23s?
IDK;
2.94s would get you 65=2400 and a starter of 9.08 still plus 20% over what you have. I like that better, but if you have to downshift to climb, then 65=3340 in third gear. That ain't so bad, if you don't have to be in it all day. But if you do, then, it will beat up/heat up, the oil pretty bad.
 
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That would point to a slightly taller tire, with a rollout of 84 inches or 26.76 tall,
like ~215/75-14, or maybe 225/70-14

If you have a 64 cluster in there, then you have a Commando box, my favorite.

Commando! I love that!! Very manly. And my wife is always encouraging me to go commando.:rolleyes:

The previous owner put 205/50R17s on the car. Speedometer is off by 10%. Odometer is off by 8%.

This 2.45 rear end is a drag. I'll keep researching, and at least keep an eye out for what ever the best solution for me ends up being. Maybe I'll stumble over the parts while I'm learning how to mig weld, fix the windshield washer, dash lights, and installing the 12 volt plug for the wife's heated motorcycle jacket (more top down time!). They say even a blind hog finds an acorn once in a while.
 
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