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).
Cone type (is not rebuildable). Look for springs between the spider gears.
Open (non-limited slip) - for reference.
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.
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