1984 Jeep CJ-7: Re-Gearing

CJM1971

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Joined
Jan 24, 2017
Location
Raleigh, NC
I have a 1984 Jeep CJ-7 with a 3 speed automatic transmission. I recently dropped in a new 360 motor, and it links up to the existing 3 speed nicely, but I'm quickly realizing I could use a 4th gear or maybe I just need to re-gear the jeep. I really don't want to install a new 4 speed transmission and go from automatic to standard shift. Any suggestions on what gear ratio I should go to and I'm assuming this would need to be done in the front and rear....no clue?
 
What rear end ratio do you have now? With regearing and keeping the 3 speed, you'll either gain low end acceleration or top speed, not both. Which do you value more? Yes, do front and rear if you want 4wd.

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Do you know current ratio?
Current tire size?
Pinning to keep those tires?
 
Even if you were to swap in any common 4 speed manual, you would still have a 1:1 top gear. And most of the 80s CJs were geared for the highway from the factory (2.72 or 3.31 with the 6 cyl). 4 cylinders got 3.54s or 4.10s. Of course, the gears could have been changed by now.

What RPM are running at what highway speed?
 
I care more about top end speed since this is a cruiser for my family, won't take it wheeling except for driving on the sand in the OBX, but other than that, it's all local driving. I have no clue what it's geared for today, when I ran the VIN it came back as a 6 cylinder out of the factory, but the vehicle has passed hands over the years. As for RPM today, without a tac, I have no clue, but I do plan on having the wife run side by side down 540 to get an idea as to what my speeds are running today with the current gearing. It has 33 inch tires and I plan on keeping it at that, at 46 years old I'm getting too damn old to climb up into anything bigger. I would like to throw a tac on there so I know what RPM's I'm running with the new motor, probably wouldn't hurt. I don't do a lot of highway driving, but for piece of mind it would get nice to know I have the ability to hit the highway and not feel like I'm doing 55 mph top speed.
 
Get a tach on it first, $25.

Then, look on the axles for a tag, will show ratio if it's still there.

Then download a GPS speedo app on your phone or tablet and go for a drive.

Once you know what you have now, then you can adjust.

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Thank you all for the replies, my speedo is WAY off, and I'm only running 33's on this old gal so my guess is it's probably time to start swapping out some gauges and then add a tach. I took the above suggestion from BenXJ and a_kelly, downloaded the GPS/Speed App and hit 540. Speedo was reading 53-55'ish but the IPhone was showing 72 mph and still rolling on. I still think with that 3 Speed Trans bolted up to that new 360 motor I should install a tach. Anyway, thank you all again! Anyone going to the Jeep Mutiny in Myrtle Beach in a couple of months?
 
Hard to imagine complaining about lack of top end in a Jeep CJ .... I'm skeeered when I get going 65 in mine.
To the original poster, you should be able to buy a relatively inexpensive tach that can mount on your steering column.

You can also see what your gear ratio is by chocking the front wheels and jacking the rear end up, turn the rear wheel once while counting the number of times the driveshaft turns... helps to mark the two.
If you have a locker or limited slip, you need to get both wheels off the ground, 2.75 turns of the driveshaft= 2.73:1
If you have an open diff the other side will turn so you'll need to keep one wheel on the ground and the driveshaft will go around half as many times (example 1.75 turns for a 3.55 ratio)
...and don't forget to put it in neutral :)

Edited to correct my open diff mistake
 
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No open diff you'll need to rotate one wheel twice to get the ring gear to rotate once. And it might be easier to rotate the drive shaft and watch the wheel rotate.
 
I'd be willing to bet you have 2.73 gears in there, being an 84 CJ-7 with 6-banger.
3.55 would be a nice with 33's. To swap gears, you'll need a new carrier in rear to replace the 2.73-only carrier in the AMC20 axle. Up front in the Dana 30, just ring and pinion gear swap, carrier should work up to 3.55 (3.73 and up would be new carrier).
 
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