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3.92's axle gears in AMC20/D30 combo?

MomoJeep

MomoJeep

Basic User
City
Seattle
State
WA
I know this is splitting hairs, but if I could pick a ratio for my set up, it would be 3.92. Yes, 3.73 or 4.1 are very close. But 3.92 would be ideal. I think 3.92 is available for D30's but not for AMC20, does that sound right?

For example, D30:

Dana 30 Yukon Ring & Pinion - 3.90

Fits standard rotation Dana Spicer 30 without crush sleeve 7.20" ring gear diameter 26 spline pinion 10 ring gear bolts 3.90 ratio Yukon Ring and Pinion sets give you the confidence of knowing
eastcoastgearsupply.com

But can't find any for AMC20 - And I'm assuming since it's not a hugely popular axle, nobody bother's making the 3.92 gear option...(?)
 
I don't think I have ever seen that for an amc 20.
 
Factory options for the AMC 20 wide track... narrow also never had them.

Code Gear Ratio Diff Type
Wide Track AMC 20​
D 2.73 Open
DD 2.73 Trac-Lok
B 3.31 Open
BB 3.31 Trac-Lok
A 3.54 Open
AA 3.54 Trac-Lok
H 3.73 Open
HH 3.73 Trac-Lok
C 4.10 Open
CC 4.10 Trac-Lok
 
4.10 is so close to 3.90, go with 4.10 :twocents:

The slight RPM increase from 3.9 to 4.1 would be negligible in the real world, in a Jeep CJ :twocents:
 
Rear ends and ratios.............Great Saturday talk

Trust these guys on here, they know they're doing!
 
Thanks everyone! Yeah, I don't need 3.92's. .. The choice is between 3.73 and 4.10. I think that's about a 200RPM difference at cruising speed... I know lots will say "just go 4.1". But here's why I'm undecided:

Note: I run 33" or 32" times, probably 33's from now on...

- I *don't* need the lowest axle ratio for off-road purposes. And, my plan would be to go with a 4:1 D300 or one of those Klune type boxes to get that off-road crawl ratio.
- I *do* enjoy cruising at highway speeds (65-70mph) on long road trips. I'd like my engine noise to be low.
- I'd like 5th gear to be somewhat of a fuel-saving option it cruising speeds, not a gear I *need* in order to go 65+
- I mostly only feel the need for lower gears when towing my small boat (2000lbs - the max the Jeep is rated for), especially starting in first gear. I *think* going from 3.31's to 3.73's would be enough to enhance the towing (a 13% lower ratio). QUESTION: do ya'll that's enough of a ratio change to feel a difference when starting in first gear towing the max rating of 2000lbs ? I want it to feel "capable" not "straining" to pull that weight when starting in first. It currently feels a bit straining.
 
MomoJeep said:
Thanks everyone! Yeah, I don't need 3.92's. .. The choice is between 3.73 and 4.10. I think that's about a 200RPM difference at cruising speed... I know lots will say "just go 4.1". But here's why I'm undecided:

Note: I run 33" or 32" times, probably 33's from now on...

- I *don't* need the lowest axle ratio for off-road purposes. And, my plan would be to go with a 4:1 D300 or one of those Klune type boxes to get that off-road crawl ratio.
- I *do* enjoy cruising at highway speeds (65-70mph) on long road trips. I'd like my engine noise to be low.
- I'd like 5th gear to be somewhat of a fuel-saving option it cruising speeds, not a gear I *need* in order to go 65+
- I mostly only feel the need for lower gears when towing my small boat (2000lbs - the max the Jeep is rated for), especially starting in first gear. I *think* going from 3.31's to 3.73's would be enough to enhance the towing (a 13% lower ratio). QUESTION: do ya'll that's enough of a ratio change to feel a difference when starting in first gear towing the max rating of 2000lbs ? I want it to feel "capable" not "straining" to pull that weight when starting in first. It currently feels a bit straining.
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Oh.. and.. I think 2200 RPM is an ideal cruising RPM..
 
I have 4.10 in my ’82. I ran 31” tires, doubt I would want higher (numerically lower) gears than that. I am planning to run 35” after I am done rebuilding it, I am thinking of going with 4.56 at that point, but with a 6.2 LS, the 4.10 ratio will probably be fine too. (Planning to run a D44 front, D60 rear)

I vote 4.10s. If spending the money to change gears, might as well make it worthwhile.
 
With all that you’ve thrown out there, my vote is 4.10. I run 4.09 gears, essentially the same, with 33’s.
 
I advise 4.10 gears. If you find that the cruise RPM’s are too high, you can easily adjust that by going to a taller tire. But 4.10 is the sweet spot for 32/33” tires. 3.73 for 31” tires.
 
Randyzzz said:
I advise 4.10 gears. If you find that the cruise RPM’s are too high, you can easily adjust that by going to a taller tire. But 4.10 is the sweet spot for 32/33” tires. 3.73 for 31” tires.
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Or going slower.
 
What engine and transmission are you running? This will make a huge difference.
 
Ok. So if your signature description is correct you are running a 258 w/ an AX15. I personally have a Chevy 5.3L with Ax15 and 3.73 gears. With the 5.3 it is ok at 70 mph. It cruises fine but I certainly don't have that much additional passing power. Even with the V8. I had a MPI 258 before this and freeway cruising at 2100 rpm was terrible. With the 6 I wouldn't even consider purposely gearing it that way. As others have said, go with the 4.10s. They will put you in the 2300-2400 rpm range (depending on ACTUAL tire size) at 70 mph. 2100-2200 is just a bit too low for the 6 cyl. You are driving a brick down the road. You need those few extra hundred rpms especially with a strong headwind. Stop overthinking this, go with the 4.10s. You won't regret it. :cheers:
 
Bit of an old thread but I'm looking at a similar situation.

Just bought a project scrambler and it came with 4.10 gears in the dana30 and amc20 (Both with detroit lockers). Previous owner said he was planning to run a stockish tire at 31" or 32".

With a modified AMC 360 putting out say 350hp, would I be happier with 33" or 35" tires on the road? I was going to utilize 32-33"x 12.5" originally, but wouldnt mind 35" if I wouldnt be slugging it around town. I've been looking at RPM charts and calculators that factor in transmission ratios but still can't get a clear picture with an unknown torque curve on the 360 engine. (Have a T5 transmission right now, but I assume that will be swapped for another manual 5 speed)
 
I have 3.54's in my axles with 32x11.50 tires plus a 383 SBC and a TH350 trans. I think of it as the 🏝️"island gear"🏝️ setup. Its kinda laid back but will certainly get up and go if it must, plus it will happily go down the highway at a reasonable RPM. But mostly, its kinda laid back. 🌴
 
DOJOLOACH said:
Bit of an old thread but I'm looking at a similar situation.

Just bought a project scrambler and it came with 4.10 gears in the dana30 and amc20 (Both with detroit lockers). Previous owner said he was planning to run a stockish tire at 31" or 32".

With a modified AMC 360 putting out say 350hp, would I be happier with 33" or 35" tires on the road? I was going to utilize 32-33"x 12.5" originally, but wouldnt mind 35" if I wouldnt be slugging it around town. I've been looking at RPM charts and calculators that factor in transmission ratios but still can't get a clear picture with an unknown torque curve on the 360 engine. (Have a T5 transmission right now, but I assume that will be swapped for another manual 5 speed)
Click to expand...
Forget the T5. There are other way better swaps. An AX15 is a very easy swap for the 360 and will hold up fine with a decent driver. The AX15 has an overdrive of .79

33s with the .79 5th gear at 70 mph would be about 2300. I would say that is a great all around setup.

35s with the .79 5th gear at 70 mph would be a little shy of 2200 rpm. In my opinion that is a little on the low side, but would probably be fine.

People have a tendency to get too conservative on the ratios and think they are going with too low of a ratio (numerically higher). They are afraid of revving too high. Trust me when I say this....It's better to be over geared that under. It's not good to lug your engine. I'm going with 35s next and going straight to 4.88s which will put me at about 2600.
 
Thanks for the input. Basically I'm stuck with 33s since the dana 30 is a bit weak with reduced tooth count on the 4.56 and 4.88
 
DOJOLOACH said:
Thanks for the input. Basically I'm stuck with 33s since the dana 30 is a bit weak with reduced tooth count on the 4.56 and 4.88
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I'd say go with the tire you want! If you break your D30 then you have a fantastic reason to upgrade to fit the tires you like. :)
 
My stock T5 seized up while going 70MPH (running 31" tires and 3:31 gears at the time). A scary experience for sure. I'd swap it out.

And one thing I think I've learned from this thread and some experience, there's not much of a difference between 3.92 and 3.73, or between 3.92 and 4.10's. And since torque curves look more like gentle hills that mountain peaks, I wouldn't worry too much about trying to nail a certain RPM, especially when you don't know the actual torque curve anyway. With that said, the low 2000's seems to be where the stock 258's sit. Any V8 in general is probably higher... mid-to-high 2000's? maybe shoot for 2500+ish?
 
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