Re-gear 1979 CJ5

[pwalsh1487]

Previous owner installed 2” lift, 33” tires, swapped factory 3 spd tranny for a 4 spd T-18 & rebuilt original 304 V8. They left the original 2.73:1 gears in the AMC20 rear and Dana 30 front ends. I‘m thinking about re-gearing to 4.10 ... still a newbie so would welcome input, suggestions on what equipment to use (make/model/etc). Thanks!

 

[hole]

Welcome. Have you played around with a gear ratio calculator? How do you use the old girl? Are you thinking about lockers, or LSD's? I ask because it'll help some with suggestions . Food for thought, going to 4.11 from 2.73 will jump the engine RPM's 1000 at 70 mph. If back roads and off roading are the majority of use, easy call. If highway speeds are the majority, might need to pause and think for a minute. I've been paused for 3+ years on this issue trying to decide on how conservative/aggressive I want to go. Mine see's it's fair share of off road, many days at a time usually, but it's at least 3+ hours (each way) of highway to get to the offroad.

 

[007]

What hole wrote is correct...I use Grimm Jeeper's site - http://www.grimmjeeper.com/gears.html to calculate drivelines and tires.

That said, I just used a CJ7 with T18 and 4.10's, but it had a 258 and 35's. It was actually a good setup, but it struggled a bit on a highway with some grade to it. I felt that 33's would have been a better fit - which falls in line with what you are considering. I think a 304 would have helped this Jeep out a bit on those grades, but not exponentially. The 304's handle highways better than the 6's (both in stock form) but again, it's not a "crazy" improvement. You need a 360 to see a major improvement.

My guess is that you'll be fine with what you propose. Your next step back is 3.73's and you'll see on the gear calcs that it's a marginal difference. Doing 4.10's now gives you room for 35's in the future should you ever decide to. I've learned the hard way on that...not planning for the future or acting as if my needs would never change.

 

[Lumpy Grits]

OP, what/how will you use the Jeep?
Do you drive lots if hills etc.
If you do this R&P change. I strongly suggest you go to LSD diffs in the front and rear.
4:10 is not a bad choice...
Gary

 

[pwalsh1487]

MANY thanks to all! Mostly country backroads, 2 lane blacktops, gravel roads and the occasional run to the gas station on a two lane county highway with a 55 mph speed limit. I’d probably delay the decision even a bit longer if I could find a 23 tooth speedo gear made for the CJ5 so I could get a tad more accurate on my speedometer & odometer with the existing set up. Also, from what I understand, going to the 4.10 would improve overall performance & gas mileage as well as getting it closer original factory spec performance?

 

[Lumpy Grits]

Contact Willys Works in Az. for the speedO gear.
The sooner you get the R&P change done. The better it is for your driveline.
Gary

 

[Posi]

They left the original 2.73:1 gears in the AMC20 rear and Dana 30 front ends.

2.73 gears are not original to your CJ. The original standard gears were 3.54.

 

[JR74CJ5]

I have 4.27's with 33's and no OD.

Runs fine on highway and I get my low for trail off the SM420 granny gear.

Not saying a OD wouldn't be cool for highway though...

 

[pwalsh1487]

2.73 gears are not original to your CJ. The original standard gears were 3.54.

Thanks Posi! I did the old jack it up, put it on jack stands, use chalk marks and count the number of times the drive shaft turns and came up with 2 3/4 turns several different times - that’s where I came up with the 2.73 and just assumed the were original to the Jeep based on what else the PO did (or didn’t do).

 

[Lumpy Grits]

There should be a letter code stamped into the rear diff housing, just to the right of the differential cover.
Gary

 

[pwalsh1487]

There should be a letter code stamped into the rear diff housing, just to the right of the differential cover.
Gary

Gary ... I’ve cleaned & scrubbed the cover but don’t see a letter code ... photo attached. Thanks!

 

[Lumpy Grits]

It's just to the right of the cover at 3 O'clock on the flat machined section.
It will be 1 or 2 letters.
Gary

 

[pwalsh1487]

It's just to the right of the cover at 3 O'clock on the flat machined section.
It will be 1 or 2 letters.
Gary

MANY thanks Gary ... found it and I just have to figure out what that 41 year old stamp is!

 

[007]

Well...this will only tell you what it came with. Not what it has. If you counted 2 3/4 then you have 2.72s.

Anything else would have been MORE than three revolutions. 3.31/3.54/3.73.


Sent from my iPhone using Tapatalk

 

[Lumpy Grits]

Looks like 'AA'.
A means 3:54 R&P. The second A indicates a Limited slip differential (LSD).
The only way to confirm this is still the gearing. Is to remove the diff cover and see if the ring gear is marked with a tooth count.
Gary

 

[pwalsh1487]

Ga

Looks like 'AA'.
A means 3:54 R&P. The second A indicates a Limited slip differential (LSD).
The only way to confirm this is still the gearing. Is to remove the diff cover and see if the ring gear is marked with a tooth count.
Gary

Gary ... that’s exactly what my eagle eyed, much younger son said ... AA!, Since mine is a 79, I thought it was a Narrow Track & the AA on a narrow track would indicate 2.73? I did get back under it again this afternoon, marked the shaft & tire, and instead of turning the tire one full rotation by hand, I turned the shaft by hand until the tire made one full rotation. I did that 3 times in a row & got 2.73 each time so I’m thinking 2.73?.? Really appreciate all your help & input!!!

 

[Posi]

Since mine is a 79, I thought it was a Narrow Track & the AA on a narrow track would indicate 2.73? I did get back under it again this afternoon, marked the shaft & tire, and instead of turning the tire one full rotation by hand, I turned the shaft by hand until the tire made one full rotation. I did that 3 times in a row & got 2.73 each time so I’m thinking 2.73?.? Really appreciate all your help & input!!!

All CJ5s are narrow trac.
What you have is a set of CJ5 axles from a 1981 (if in fact you have 2.73 ratio AA).

 

[pwalsh1487]

All CJ5s are narrow trac.
What you have is a set of CJ5 axles from a 1981 (if in fact you have 2.73 ratio AA).

Posi ... thanks for (possibly) solving the mystery!

 

[Posi]

Posi ... thanks for (possibly) solving the mystery!

Just to clarify,
The 79 axle never came with a AA code. A was 3.54 (standard) N was trac loc for 79
What you have is an axle housing from '80-81 that should have 2.73 gears, could be a NT 50.5 or WT 54.5 (rear axle) you will have to measure.
NT ran to '83 in a CJ 5, WT CJ7 ran '82-86
I hope this helps.

 

[Lumpy Grits]

Time to remove the cover and check the ring gear markings.
Sorry for my error. I did in fact look at the wrong chart.
Gary