Differential Ratios For Imperials

Imperial Home Page -> Repair -> Rear Axle -> Differential Ratios

Question from Charles:

I would like to change from a "non-sure" grip rear end to a "sure-grip".  Could you tell me the foolproof manner. I think you have to grind the axles some. Please give ALL the steps as I have never been into a rear end of any kind.

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From Pete:

I've done this swap a number of times on 8 3/4 rears and it's not hard. No grinding necessary. 1) Remove wheel covers, wheels, brake drums, axle shaft retainer plate nuts 2) Pull axle shafts out of housing being careful to avoid damaging oil seals 3) Drop drive shaft 4) Remove 10 or 12 (or however many) nuts securing pumpkin to rear axle housing 5) Place drain pan under housing and remove pumpkin. Install whatever pumpkin you want and reverse steps to reassemble. Make sure to set axle end play after installing axles or you will wipe out the rear wheel/axle bearings. Add sure-grip lube and you're done. 

From Wayne:

Be careful with axle swaps on 63 and earlier sure grips. the sure grip axles are 1/8" shorter than the open units, they cannot be interchanged.

How you can tell if you have a sure-grip differential:

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From Wayne:

There was an ID tag on one of the center housing bolts, it is round and says "sure grip" if the unit has been apart or altered the tag is probably gone.

From Larry:

Jack one side of the car until the tire is free to turn. If it will not rotate with the trans in neutral (brakes released), the differential is a (sure grip) (limited slip). If the wheel rotates it indicates an 'open' differential.. Like Ken said; these are VERY strong units and will take much abuse...with or without the limited slip feature... While on this subject! Any 8-3/4 rear end will accept the limited slip feature, either the disc type or the cone type will fit these popular 8-3/4 rear ends. The 69 and up cars used the '489' (last three numbers of the casting number) and the 68 and older cars used the '742' case. I don't know when the '742' series began. (about 1966/67) Earlier cars had another version and will accept these same limited slip sections. Three different pinion designs were used, 1-3/4, 1-7/8 dia. and the tapered pinion version used in the 1969 and up, '489' case. Small cars (some B Body and most A body) used the '741' case, and I think the same limited slip will fit that light duty, 1-3/4 dia. pinion, rear end.

Question from Jeff (1967):

My brother needs a new rear end for his 67 Imperial (or at least parts) I know where I can get a rear end out of a 71 imperial for free. Would they interchange? Would there be any reason not to  interchange them? Would it be better to fix the one in there now? 

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From Pete:

The 8 3/4" differential or "pumpkin" assembly fits a whole ton of years and models. Old style rears (with the big nut on the end of the axle) will interchange too. You can even put the pumpkin from a 70 Duster into an Imperial, but I wouldn't recommend it since pinion shaft diameter varied. Going from a 71 to a 67 should be fine. Just be sure to set the axle end play during installation (see Factory Service Manual) or you will wipe out the rear wheel bearings.

From Steve:

Your brothers' Imps entire gear-set can be removed from the housing w/o removing the rear-end assembly from the car. First the axle shafts must be slid out each side which is not too hard, then unbolt the driveshaft. While unbolting the drive shaft, look at the rear end and you will see a series of bolts in a circular pattern...remove them and slide the whole gear-set out towards the front of the car. Be careful, it is heavy. I would repair it or replace it with the '71 gear-set if it is in better shape...but you are uncertain of it's condition. Just remember that if the ratio is different, your speedo will be incorrect, then you will need to replace the speedo gear with the correct one. The U-Joint should interchange but I would call an auto parts store and see if they take the same ones or measure the yokes just to be certain. Are either of the rears Sure-Grips? His is called a 8-3/4 "Banjo" or "Front-Loader" type rear because the gear-set can be removed towards the front of the car while leaving the housing still bolted in. The other type is the 9-1/4 "Carrier Tube" type which can be identified by having a stamped steel cover facing the rear of the car, which was used in mid-70's Imps and NYB's. The 8-3/4 rear was first used in 1957 in which there are 3 types: small stem, larger stem and tapered pinion. There are also TWO basic 8-3/4 rear axle assemblies. One has tapered axle shafts, with the brake assemblies attached to the axle shaft with a keyway and a large nut. This unit was used up to about '64. Since '65,it has flanged axle shafts. The 3 ring and pinion center sections ARE INTERCHANGEABLE between the two axle assemblies. The 3 different ring and pinion assemblies can be identified by a part # cast into the center section on the drivers side of the case. The small stem has two #'s:1820657 (up to '64) and 2070741. The large stem unit is 2070742. The tapered pinion(2881489) was introduced in '69 to replace the large stem. Another way to identify them is: Small stem = 1 3/8 pinion shaft dia. Large stem = 1 3/4. Tapered = 1 7/8. He might want to replace the seals while it is apart.

Question from Kerry (1973):

What is the standard rear end ratio for 73? My build sheet shows 3.73 but I THINK it was an optional ratio. At 70mph the engine seems to be turning higher than it needs to. A lower ratio would give better highway mileage as the cost of performance around town. I have access to another rear end with an unknown ratio that I may swap out. When I did my rear brakes, I noticed that the drivers side axle had considerable play in it. It would rotate maybe 15 degrees, i.e. 12 o'clock to 1:30 or so and had a clunky sound when it stopped. The passenger side had no play. Is this normal?

Reply from Dick:

Wow! That sounds like really excessive play to me. Are you sure you just weren't feeling the action of the lockup clutches (assuming you have a posi rear end)? To check your rear axle ratio, jack up one side of the car (use a jack stand of course) and slither under far enough to put a chalk mark on the drive shaft. With the car in neutral (the front wheels blocked, of course) and the parking brake off, rotate the wheel that is off the ground two full turns, while watching the chalk mark on the drive shaft. Count how many turns the drive shaft turns in two complete wheel rotations, that will be your rear axle ratio within a few percent. If you have posi-traction, you'll feel some clunking and hear some noise when you do this, not to worry, that's normal. It's easier, if you can find it, to see the little metal tag on one of the bolts on the pumpkin case, if it is still there. It will be two numbers, one in the 40's, one in the teens. The ratio of those two numbers will give you the rear axle ratio directly. (The numbers are the number of teeth on the ring gear (the larger number) and the pinion.)

Follow-up from Kerry:

I don't have posi. The play seems a lot to me also. What would be the probable cause? I haven't seen a tag on the chunk and have not yet measured the ratio by turning the wheel. I'm going solely on what the build sheet said.

Follow-up from Dick:

You're real, real sure nothing else is turning, right? If the wheel will turn from 12 o'clock to 1:30 o'clock without any other shaft moving at all, there is an enormous amount of play in the gear train. Taking 41 as the approximate number of teeth on the ring gear, number of degrees of wheel rotation from one tooth to the next is 360/41=8.8 degrees. 12 o'clock to 1:30 is 1.5/12X360 degrees, or 45 degrees. So what you are seeing is between 4 and 5 teeth of ring gear rotation. This cannot be completely the fault of the ring and pinion setup because the car would not drive that way. Thus there must be a major breakdown in the spider gear setup, either a spider gear has come out of it's holder or the cage has come apart. I think you are about to witness a pile of parts on the road under the car, and pretty darn soon, too!

Question from Roger (1974):

I think the gear set in a74 Imperial  is 3.23/1 and a75imp is 2.75/1 can I put the 75 gear set in a 74 and what problems will it cause beside speedometer? 

Reply from Steve:

To be certain what ratio the gear set is.....unbolt the cover and rotate the ring-gear by either turning the pinion or an axle. The ratio # is stamped on the ring gear, on the outside of it. You'll have to look sideways at it. No damage will be done to your car by using a different ratio other than an inaccurate speedo. To rectify that....find the car that the rear-end came out of....unbolt the speedo cable from the tranny...and pull off the plastic speedo drive gear. If you are unable to find the parts car, try going to an auto parts store and tell them the new ratio and that you need a drive gear for a 727 Torqueflite.

Question from Gared (1981 - 1983):

Would changing the gear ratios (on an EFI Imperial) be a difficult thing to do? What would I  need  to buy, a whole new rear end? 

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From Frank:

I don't know about Dick's 81, but my '83 Imperial has the 8 1/4 inch rear axle. The 8 is a good, solid rear and can take quite a lot of abuse. The 7 1/4 inch is a piece of junk and a lowly 2 BBL 318 can easily blow them apart. The 8 1/4 and the 7 1/4 both have ten bolt covers, but the 8 1/4 has 3" axle tubes while the puny 7 1/4 has 2.5" tubes. My (now defunct) '78 LeBaron has the 7 1/4 rear with a 318 2 BBL. I put in at least four 7 1/4 rears (three with broken gear sets, and one with a broken axle tube) before I realized my complete & absolute stupidity. I got smart and put in an 8 1/4. Never had anymore trouble. (All of those replacement units were from the junk yard, and I DID inspect all of them for damage.) If you have a car with a 7, there is an 8 that will drop right in. The drive shaft was a different length because the nose of the differential is different. Get any correctly sized unit from the junk yard, but don't use it. Most junkers bend, or otherwise distort the shaft. Take the drive shaft and have a new one made. They can be cheaply fabricated. (Look in the Yellow Pages for "Drive Shafts".) The fabricator will use the old yokes and weld a new tube to them. A quick balance and better than new! BTW, that 8 is a much more massive unit. I'm a pretty small guy, but can easily handle a 7 1/4" rear alone. However, getting that 8 1/4" into the trunk, back out of the trunk, and onto the springs (installation) was and entirely different matter!

From Dick:

I'm in a little over my head here, you should probably talk to someone who  knows performance Mopar facts. The Mopar Mailing List has a group of such guys, but if you  are not a member, perhaps someone on here can tell you.  I do know that you at least have to change the ring and pinion gears, and I  suspect the easy way is to find a similar car with a higher gear ratio in a  junkyard (Mirada or a Cordoba, those are built on the same platform), and  simply exchange the whole rear end assembly, drum to drum. Your car will  have the same differential as a Cordoba or a Mirada (8.25" ring gear, 3"  axle tube, 10 bolt cover), but a Mirada or a Cordoba may have the 2:71 or  2:94 ratio ring and pinion. A check of the data plate will tell you the  axle ratio, but you need a magic decoder ring to know what you are looking for.  That will be a simple bolt on exchange.

Question from Phil (1981 -1983):

Recently, member Dick Benjamin stated that the ratio was 2.20:1. No wonder I can't get the tires to chirp. Seriously, I think I'd be a lot happier with 2.7 or 2.9.  I've never done anything to a rear axle...how's this done? Is it better to swap axles? What would work?

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From Bob:

There is a kit, PN 4205869, for gears and pinion, for the following ratios: 2.24, 2.26, 2.71, & 2.94 - they'll all fit into your case.

From Matthew:

If I'm not mistaken , these years use 8.25 inch differentials.... I'd look for a Diplomat cop car Sure grip diff, measure( I bet the dimensions are the same) and swap (take the rear sway bar too...)

From Rob:

When I bought my '83 it came with a posi. I was told it was a Mopar performance posi w/ a 2/94 ratio in the stock rear.  I believe they just sell the center section and you have it installed. In all honesty I have barely looked under the car & it allegedly has Diplomat cop car springs and a rear sway bar ( it's kind of small-OK I looked once).

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