Mirada rear end

J Body General Discussion

  1. Mn mopar

    Mn mopar Well-Known Member

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    Yeah where is the code that tells me what rear end is in my 1980 mirada i know it is a sure grip but other then that cant figure it out? Only asking so i can get a better gearing on it, opinions are accepted for the final drive
     
  2. volare 77

    volare 77 Well-Known Member

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    Look under the car. 10 bolts 8 1/4 9 Bolts 7 1/4 . If the rear has never been apart there should be a tag on one of the bolts for the axle ratio.
     
  3. Mn mopar

    Mn mopar Well-Known Member

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    Awsome good to know
     
  4. BudW

    BudW Moderator Staff Member

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    Ma Mopar quit installing differential ID tags in the early/mid ‘70’s – so if you find a differential tag (under one of the inspection cover bolts) on any FMJ vehicle, then someone added it, after the fact.

    What they used at the factory was paint stripes on the differential housing to indicate what gear ratio and to indicate if it was limited slip, or not. On top of that, the paint was added to untreated steel/iron – so after a short time, the paint falls/wears off.

    It would be nice if this data was stamped on the fender tag – but it is not.
    The fender tag was mostly for how body was to be built before the shell got painted (so paint type, trim type, 4-speed hump, etc.).

    The only way to get that data is to find a broadcast sheet (build sheet) inside of the car somewhere – which even then, that is not easy to find.
    The broadcast/build sheet is the blueprint of how your car is/was optioned out.

    For the other 95% of us with FMJ vehicles (without build sheets), the only way to know what gear ratio you have is to either remove inspection cover and look for the stamping on ring gear (by the way, Chrysler recommends you change your differential lube every 30k miles anyway - but hardly anyone does) OR jack rear wheels off ground (be sure to use jack stands!) and count tire rotation while rotating propeller shaft. A person can also determine if you have a limited slip differential, at same time.
    By the way, I would say that 99% (which is a guess based from experience) of all FMJ differentials were not built with limited slip. A person has to either special order it or have it installed after purchase.


    Almost all 7¼ differentials have 9 bolts attaching the inspection cover and cover looks somewhat diamond shaped.
    All 8¼” differentials have 10 bolts and cover is oval shaped.

    All J-body 7¼” differentials – the tubes taper down just before going into the center section (from 3” diameter tube to 2½” tube – which is what I look at too quickly determining differential types.

    I would also guess (no numbers to back this up, either, but based off experience) that 75% of all J-body’s have the 7¼” differential vs. 25% having the 8¼” installed.
    BudW
     
  5. volare 77

    volare 77 Well-Known Member

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    Yea, I couldn`t remember when they stopped putting the tag on the bolts. I assumed it was later then that.
     
  6. Mn mopar

    Mn mopar Well-Known Member

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    Ok so its been a while but i finally got under the car the other day and i do have a 8.25, 3 questions, #1 2.45 is standard gear ratio for a 8.25 sure grip or something like that? #2 what would be a good ratio to run to wake the car up a bit? #3 any recommendations on where to acquire a new set of gears? TIA
     
  7. BudW

    BudW Moderator Staff Member

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    A differential gear change will make most people’s cars wake right up and make ‘em fun to drive, again. In many cases, more bang per buck than what engine modifications can make.

    Depending on year vehicle was made, the 2.45 might be standard. in the early '80's, it changed to 2.21 (which makes it a dog).

    For FMJ vehicles, the available gear ratios for 8¼” differential are: 2.21, 2.45, 2.71/2.73, 2.93/2.94 and 3.21/3.23.
    Most police cars came with 2.93’s.

    The same differential was also used in Dakota, pickup, van, Jeeps and B, C & R-bodies.
    In pickups, 3.55 is maybe most common, with some at 3.73 and at 3.91. I think 4.10 might be an option.

    Jeeps also come with a 3.07 (which was never installed in a Dodge).


    My opinion for a street driven (318 automatic) car are:
    3.23 is great for both in town and highway.
    3.55 is great for in town (mostly) or 2.93 for highway (mostly).
    3.55 (or 3.91’s) are really fun for city usage – but fuel mileage might not be as much as you would like (higher engine RPM).

    Most people do not notice any difference in fuel mileage between 2.21 to 2.93’s.
    A lot more noticeable power with 2.93 or lower gear set (higher numerically gear set).
    2.2’s and 2.4’s gears are absolute dogs to drive.

    For racing – I can’t comment on gear set (it depends).

    There is another member here who can do the math, using your engine size, transmission type and so forth – to determine best gear ratio. What I listed above is based off experience, not math (I’m too lazy to do the math).


    Best Source for gears: lately has been eBay.
    3.55 and 3.91’s is common to find (factory or aftermarket). I do find 3.23’s every so often.
    Here is a couple I found:
    https://www.ebay.com/itm/Mopar-8-1-4-Rear-End-2-94-to-1-Ring-and-Pinion-Set-Dodge-Plymouth-8-25/302860993494?hash=item4683ec03d6:g:VFIAAOSwwRtbXKSk
    https://www.ebay.com/itm/Genuine-OEM-Dodge-8-25-Ring-and-Pinion-3-21-gears-Ram-Dakota-Durango-Cherokee/112479610739?hash=item1a304ea773:g:6WgAAOSw5cNYQBeS
    I would not be afraid of using a used gear set. Check for broken teeth and for rust – if neither is present then I say to go for it.

    As far as I know, ALL 8¼” ring & pinion rear gear sets are the same. The front gear sets (4x4's) will not work, nor will independent rear suspension gear sets

    I have worked on more differentials than many people have ever laid eyes on, so to change is not that difficult of a job - to me. That said, if you have a friend who has performed differential work before, invite him over (beer and nacho’s make for a great invite bait). Otherwise I would take gear set (and a new bearing set) to a differential shop for replacement. There are some special tools needed that most people don’t have access too. It takes about 4 hours to dissemble, replace bearings (and gears) and reassemble.

    Note: anytime a differential is apart – I highly recommend installing a limited slip differential at same time. Not much additional work and you will be needing it sooner than you think.


    There are 3 different kinds of limited slip versions out there: The clutch type and cone type. Some jeeps use an air clutch – but I say stay away from them (troublesome, plus you need to make a ton of modifications).
    There are also two different axle splines ’68-96 (FMJ;s) and ’97 and up (27 vs. 29 spline).
    The ring and pinion gear set doesn’t matter what year - the carrier does matter on year.

    Clutch type (is rebuildable). Look for clutch notch (white arrow).
    Carrier 8.25 Clutch.jpg

    Cone type (is not rebuildable). Look for springs between the spider gears.
    Carrier 8.25 Cone.jpg

    Open (non-limited slip) - for reference.
    Carrier 8.25 Open.jpg

    My favorite type of limited slip is the Eaton "True Trak" – but they don’t make one to fit FMJ vehicles (which is a shame).


    One thing to remember is with either a tire diameter change and/or gear ratio change, you will need to change the transmission speedometer gear to correct the odometer/speedometer readings.

    The original (plastic type) speedometer gears are not easy to find, especially with unusual gear (or tire diameter) sizes. The good news is this might be a time to change over the older metal speedometer gear set – which has a wide range of speedometer gears available (and are currently being made aftermarket).


    Another area to look for, during a rebuild, it to check for wear at the axle bearings (yellow arrow). If you can feel anything with your fingernail – not all is lost. Many years ago, anything caught with fingernail was trash (this one looks like could be that way, maybe). Now they have offset axle bearings – or axle saver bearings which a person can use.
    8.25 axle.jpg

    My wife disagrees with my tactic, but I think it is good to drive a car hard in turns every occasionally, just to make sure those bearings get lubed (he he).

    BudW
     
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  8. Mn mopar

    Mn mopar Well-Known Member

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    Budw, isnt the sure grip already a limited slip? And when i do tires im going with same size already on so no need for a gear change on that, but i do think the 2.93 gears is what im looking for as i will be doing some highway driving, but when i do the gears the rear end is coming out and im going to put new seals, bearings, leaf spring bushings, sand blast it and a few coats of fresh paint then put it all back together and in, after that i move to the front for a tear down clean replace all bushings grommets links ball joints ect, doing the rear end over this winter and front end next winter just giving the old girl some loving a little at a time lol
     
  9. volare 77

    volare 77 Well-Known Member

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    I have a NOS 8 1/4 sure grip for the 2.45 ratio. PM me if interested.
     
  10. BudW

    BudW Moderator Staff Member

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    Limited slip is a universal term.
    Some terms like “Posi/Positrak”, “True Trak”, “Trak Loc”, ”Sure Grip”. etc. are all trademark names. I prefer to use more general terms when talking.
    Same goes for “Freon”. It is a Dupont trademarked name for R-12 Refrigerant, which worked so well that almost everyone calls any version of Refrigerant R-12, R-134a (and so on), “Freon” now.


    Sounds like you are on right track with your rebuild!
    If housing is sandblasted or bead blasted, be sure to put some primer down before you paint (or right after its blasted) - so your paint job will last longer.
    Also, be sure to clean out the housing very well because sand (or glass beads) will do a bad number on your new bearings (I’ve seen bad things happen that way and don’t want you to do the same).

    A tire size change will require a speedometer gear change (in many cases).
    A gear ratio change will definitely require a speedometer gear change.

    I don't what year car you have (and I do not have any manuals for '80 or 81 to look at). Here is what my '79 parts manual shows:
    Tire size:
    79 PM pg 22-3.PNG
    The speedometer gear is listed below (PDF file)

    From my '82 parts manual (which PDF file is not easy to read or to interpret)
    82 PM pg 22-533.PNG
    Again, the speedometer gear is in PDF file, below

    If you currently have a 2.45, then a 2.93 will be more fun (with a negligible difference on fuel mileage).


    Also, (I forgot until Volare77 mentioned it), the case/carrier, the part the ring gear bolts to (or item pictured in 3 pictures, post #7), is different between gear ratio 2.21 and 2.45/2.47 and all other gear ratios (and 2.71 and lower gears (higher numerically)) – so if your differential is currently a 2.45 and you wanting a lower ratio gear set, then a case/carrier replacement will be a requirement.

    The 2.21/2.45 ring gear mounting flange is mounted father from the pinion gear center-line than all the other gear sets – or otherwise the ring gear would to thin to have any strength.

    Also, it is not easy to find a limited slip carrier for a 2.21/2.45 gear set, so if a person is wanting to keep that gear set (not sure why . . .) and you wanted limited slip – that 8¼” carrier Volare77 has might be one of the few remaining ones out there.
    BudW
     

    Attached Files:

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  11. Mn mopar

    Mn mopar Well-Known Member

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    I would be but from what budw said i would need a different carrier to run a 2.93 if i read that right
     
  12. Mn mopar

    Mn mopar Well-Known Member

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    Budw i have a 1980! So your saying if i want to run 2.93 gear set i would need a different carrier correct, and if so what cars would have had the one i should be looking for, i do want to keep the limited slip as well!

    And yeah i had a buddy rebuild his rear end in high school and did not clean it very well and a week later he was putting a new one in lol

    For the fuel mileage i could really care what it gets as it currently is my weekend cruiser/ car show vehicle,
     
  13. BudW

    BudW Moderator Staff Member

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    My first question is: does your existing 8¼” have (a working) limited slip?

    If it is, and also a 2.45, then the only ratios that will fit it, is either 2.21 or 2.45’s.

    You can get other gear sets to fit, if you knew what the exact difference between the two versions were, made a spacer to fit it and find (or make) longer left hand thread hardened bolts to attach it with.
    I would guess there is ½ to ¾” difference on location of ring gear mounting surface between the 2.2/2.4 vs. all other gear ratio carriers.


    I didn’t know that your intended purpose of car was “my weekend cruiser/ car show vehicle”. In this case, I recommend either 3.23’s or 3.55’s (depending on how much highway usage it might see).
    If you are changing gears, you might as well get one closer to what you want.
    That car would be fun with 3.55’s in it, in town.

    To answer your other question, then yes, you need another carrier if going with 2.7, 2.9. 3.2 or lower gear ratio gears.

    If you don’t want limited slip, I have a good open carrier from my ’84 Gran Fury police 8¼” (2.94) – which I will be changing over to limited slip, that you can have (for cost of freight), but I don’t think you will (or anyone else will be) happy with an open differential.
    You must remember that gear ratio is like a torque multiplier. The more available torque = the need for more traction. Installing a lower gear ratio will give you more available torque – even for a stock 318 2-bbl car.

    From a dead stop, my ’77 Volare will run laps around my ’86 Fifth Ave until both get to about 30 MPH. Both cars are identical except one has 2.21 and other has 2.94 gear ratio’s. Everything else is the same.
    Putting 3.23's into both cars, is on my list (both are daily drivers).
    BudW
     
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  14. Oldiron440

    Oldiron440 Well-Known Member

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    If you people want to check speedometer for its accuracy try a GPS speedometer app for your smartphone. They work nice.
     
  15. BudW

    BudW Moderator Staff Member

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    A good point.

    Older speedometers – you can calculate/guess the correct gear needed – but sometimes calculating (or guessing) does not get you there.

    Comparing miles traveled on existing vs. actual mileage traveled is better (or using an app/GPS) to calculate the needed gear.

    If car will not be driven much, then it might not matter much – but cops love to write tickets. I would rather have them go write tickets on the foreign cars, Mustangs, etc. and leave us alone.
    BudW
     
  16. Jonnyuma

    Jonnyuma Well-Known Member

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    Jeep Cherokees are a good source for 8.25" gears and SureGrip units. You can find em in any boneyard, on ebay, or buy new. The popularity of the Cherokees w 4x4 guys means a good selection of aftermarket stuff.
     
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