Differential ratio

My 67 A 100 came stock with the slant 6 and the a727 auto trans. The PO swapped the slant 6 for the 318. Does any one know if the slant 6 and the 318 used the same differential ratio?
Thanks!

no they would not. My guess is if they did then maybe a 3:53 gear mine had a 3 speed and a /6 it  had a 3:91 i did the 318 /auto trans swap. with 3:55 rear but those are still to steep. and i'm looking for 3:23 at this time. on your drivers side wheel well should be your vin plate and the rear gears would have been stamped on there.  3.91, 3.73 , 3.53 , or 3.5 ,3.31 love to know what it says,

looks like JKR is online he may have the correct answer...

gonna step in it here. if all that was changed was the engines then the ratio of the rear gear is compatible with the auto trans and has little effect. power curves are different for sure and thats where your difference would be. i went from 3:55 to a 2:73 set to "calm" the rpm at 60 mph. had to go to a chrysler wagon to get them but i get decent fuel economy and driveable at 60, for a "brick". 3:23 will still be a bit high for highway but the 2:73 set will save you tires, it will barely spin them.

what would the 2.93 gears have been in jkr ????

Mopar put different gears into different cars all thru the 60s. All according to the intended use, special orders and what they had around!
You can do searches for different gear sets, but the freeway gears are a bit harder to find.

To figure out what gear works out for you, here's one of the many online calculators.
http://www.differentials.com/technical-help/differential-gear-ratio-calculator

With a typical 28" tire, 3.91 is quick off the line but too steep for constant freeway use.
3.55 is a good compromise if you have a taller tire and a good engine and insulation.
3.23 is the most common and a good overall gear. It won't cause you to break the tires loose just going across intersection paint stripes, but isn't buzzin at 65MPH either.
Any 2.XX gear is going to be a little sluggish off the line, but will pull on the highway great.

Having previously been the wrench monkey in a rearend shop for several years, rebuilding hot rod 9" and 8 3/4 rears, I say what Rod says.  You can fiund pretty much any thing in any car, you never know whats ACTUALLY in there.  After what...60? years? who knows whats been changed.   Do the gear test after disconnecting the drive shaft and raising the rear.  Then you will know for sure.  

My favorite way to effectively "change" gear ratio is to play with tire sizes. Less work, and can give your van a really good look after!  Larger tires are better for freeway speed, and smaller tires are better for off the line power. Like I said. Its just less work.  Anyway, thats just me.  But then again, Im happy with my 3:73 gears, it gives me a reason for big off road tires! (which is what I wanted anyway!!!)

I've got a factory original 8 3/4 with 3.23:1 and limited slip diff. My tires are 255/60R15 and the engine does 2100 rpm at 60 mph. I agree that the gears could be taller because I have enough oomph to pull off the line but I don't really see taller gears a necessity as I have found that for what I use my van for all the gears seem to fit very good. I have the A-833 manual trans prepped by Passon Performance with 3.08 First gear.

dix wrote:what would the 2.93 gears have been in jkr ????

a 2:73 gear set would have been very common in a chrysler 4 door sedan of just about any type. also dodge cars like the monaco and polara. and if you are the lucky one in a pull-a-part and find an old police car i'll bet the farm there will be a limited slip diff in it. the ratio will always be stamped on the ring gear if you get the unit out to look at it. and if not, pull your phone out, get it into calculator mode, and divide the pinion teeth into the ring gear and it will give you the ratio. the best case for a 8 3/4 rear end is the xxxx489 diff. if the gear set is junk "take it home anyway". drag racers pay well for an old xxxx489 case. stamped right on the drivers side of the carrier. and finally after all the years "richmond gear" is finally starting to make several different ratios for all the cases, xxxx741-xxxx742,xxxx489 out of the 8 3/4 family and a posi unit called powertrax. they are good !!!

With 225/70/14's, manual trans, 273v8 and 2.94 rear gears my van pulls about 2600rpm and 65mph.

I have to string out second gear to about 35mph before I shift, other than that no issues at all.

The van came with 3:55 but it was turning rpms way above my liking for highways speeds around here. At 65 I have to stay in the slow lane and hope no one runs me off the road when they pass.

I feel ya with the 3.55. My 65 with the Slant six and a three on the tree is in the same boat.
I also have a 66 with the 727 auto and the 3.55 rear end but I have not had it on the road.
I also parted out a 67 with the v8 and auto with a 3.2 rear end. I couldn't get a clean title on the 67 so I pulled everything off and scraped it. I will be putting the 318 and 3.2 rear end in my 66.

I'm looking to change the differential gears in my '67. 225 and 3 man. What is the highest gear this thing can handle, on the freeway it sounds like it is about to come apart. I believe it has 3.98. I'm thinking 3.23 or higher. Any thoughts?

Mikes, check out my post further up for an explanation of how the gear ratios affect driving and what others have shared in this thread.
Then find out which case you have and described by jkr.
Look here for new sets (PDF warning)
http://www.richmondgear.com/pdf/Richmond_Catalog_Section1.pdf#page=6
or contact Dr. Diff for a complete drop out center section.
and check out local used center sections.

It is hard to tell what motor/ trans people are running but have a much better idea of what I need. Thanks to all ya all. I think that I will go with the 3.23 gears

WoW, I have learned so much my brain went on stryke. So My Pops and I went to a PicnPull and found a '63 Imperial with the last 3##s being 742. From what I have been told, If I wanted anything taller than a 3.55, that is what I need to find that would fit my ass being a ###741 pumkin. I found and have installed a 2.93, instead of the 3.91 that it came with in a 225 with a stnd 3, 215/60/17 wheels. I have not been on the road with this yet because i'm also putting a clutch in and doing the Disc Conv. at the same time. I have a feeling that the gears are gonna be too steep but at least I will get to feel the change and go from there. I have to give a lot of thanx to you all out there that care enough to throw a few bones. I love you gear heads. I will get some pics together soon....

I'll be interested to see how it runs down the road with that wheel/gear setup myself.

with 14" rims and a 273 rear gear mine turns 2500rpm at 60-65 mph., I have to run 2nd gear out to about 30-35 to shift without making 3rd bog a little but it runs fine other than that. I actually plan to get a pair of 15' slots for the back so I can get a specific look and gain a few more rpm on the highway.

Sounds great Mike!
I will bet the you WILL notice the difference when trying to slip the clutch taking off!
A 3.91 will allow you to darn near dump the clutch, the 2.93 will require finesse to not stall.
You will also find that 65 MPH will be a lot calmer. Hope it still pulls hills around you!
Make sure you install without nicking the axle seals and adjust the axle lash properly.

So i started this post last august and had lots of responses. At this point i have confirmed by the vin plate that my 67 has the 391 rear diff in it. as it now has the 318 instead of the original 225 i will add a new rear diff to the list. my driving will mainly be open highway and some freeway in and around Minden, Gardnerville and Carson city. Sounds like the 323 is the way to go?

That's what i'm at now, did the /6 to a 318 swap, started with the 3.91 , droped to the 3.53 and droped again to the 3.23 gear now.

What driving characteristics do you see with the 3:23 and did you have to change out the whole rear diff or just replace parts in the original diff? Thanks Dix.

With the 3.53 gear the van ran a high rpm, but don't have a tach installed  but you could tell the engine was  running hard down the hiway at 70/80 mpg, plus it run to hot. i only had it out a few times since i installed the entire center section/ chunk  3.23 late in the fall, but you can tell it's running cooler and quiter in side the van with the 318 / 4brl there is pleanty of power anyways,  no problem running up hills or hiway.  Also i have a friend running a 2.73 gears in a corry cruiser i'm telling you that thing runs very well Pennslvania  is all hills   so i did not  want to gut slugish on the hills, if your running hiway and flats  the 2.73 will give you buter gas milage, most of the big  boat mopar  had the lower gears , Imperials, newyorkers polara and such, some had the sure grip (posi) and no i'm not running one yet, only if i find one at the right price, and $600. is not what i'm talking about, jkr/jeff had posted some interesting  info about aftermarket company making aftermarket limited slip center sections for around $450. i think.  Hope all this helps, and let me know if i can help,  keep it fun.. Dix...

Richmond gear powertrax units DIX.
I run 273 gear set in my truck. mild 360 with a 2bbl carb and stock 727 auto. highway at 65 mph about 2100 rpm and enough grunt to pass somebody if needed even on a hill. just need a bit more runway than a 323 gear set.

Jeff i'm going to look that up and post it under a100 parts link, hope you don't mind...

My van came with a 318 and 391 gears. Not at all fun on the interstate. Today I picked up a center section with 291 gears I'm thinking that should drop my rpm about a 1000 at 65. the old center section is a 489 and the replacement is a 741 these will interchange right?

All 8 3/4 center sections will interchange with each other. Two things to watch are 1 the driveshaft U joint size . There are 2 different sizes 2 1/8" small/ 2 5/8" large (mostly on performance cars)... The second thing to watch, if you have a Posi make sure that the spacer block does not fall out. If your installing a Posi make the spacer bock is in the gear set.
It is in the hole where the axle splines are. Easy to tell ..IF you can see all the through the splines STOP your block fell out/ or you need to get a spacer block kit. POSI ONLY issues.
With out the spacer block you will not be able to adjust the axle end play. The spacer block kit is 3pc. Two mushroom shaped spacers & one retaining pin. POSI ONLY.
With an OPEN rearend the spacer block is not needed.