3:07 gears drive ability - JeepForum.com
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post #1 of 15 Old 07-22-2021, 06:50 AM Thread Starter
yjcharlie
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3:07 gears drive ability

Hello all,
I have an 87 YJ with the 4.2 and stock tires that measure 28.5" in diameter. I have the stock axles but with 4:11 gears currently.
I want to switch to 3:07 gears for highway drive ability. That would put my motor at 2k rpm's, and thats where the peak torque is for this engine.

How bad would that hurt around town driving? I do have the ax15 which has a higher number first gear than the ba-10 that I took out.
I'm not concerned with off road performance.

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post #2 of 15 Old 07-22-2021, 07:07 AM
shaggyjim
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I had 3.07’s in mine when it was stock and they did just fine on the I-5 and I-15. I didn’t have an overdrive so I can’t speak to how it would do with that.
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post #3 of 15 Old 07-22-2021, 09:54 AM
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Most of the 4.2/5 speed units have 3.07 axles as that was the standard. They do great on the interstate and can actually use that 5th gear. As far as the BA versus the AX, there isn't enough difference in gearing to be of any importance. I think most of us add larger tires and then wish we had 4.11 axles or even larger.

Makes me wonder how you got the 4.2/4.11 gear combo? Did you or a PO motor swap, or axle swap? If you have the original D35 in an '87 it is a non c-clip axle which is something of an advantage.

Since the majority of people would rather swap in the opposite direction from your plan, you might be able to find someone willing to do a trade. I think an axle swap might be a better and cheaper way to go than to regear the axles you have.

I won't get into the whole 8.8 swap stuff, but wait a second ... a bunch of guys are getting ready to tell you about it.

Edit to add: I see you have a 2.5" lift kit. Why are you not going with a larger tire? Like 31" maybe? Still great road manners and a good matchup for those 4.11 gears.

Bill

1988 Wrangler,4.2L I-6, Howell EFI JP-1,CRT HEI Ign,AX15 ext slave conversion, Zone 2" lift + 1" BL + 1/2" booms, BFG 31 x 10.5, Bestop.
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post #4 of 15 Old 07-23-2021, 09:06 AM Thread Starter
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Quote:
Originally Posted by bpounds View Post
Most of the 4.2/5 speed units have 3.07 axles as that was the standard. They do great on the interstate and can actually use that 5th gear. As far as the BA versus the AX, there isn't enough difference in gearing to be of any importance. I think most of us add larger tires and then wish we had 4.11 axles or even larger.

Makes me wonder how you got the 4.2/4.11 gear combo? Did you or a PO motor swap, or axle swap? If you have the original D35 in an '87 it is a non c-clip axle which is something of an advantage.

Since the majority of people would rather swap in the opposite direction from your plan, you might be able to find someone willing to do a trade. I think an axle swap might be a better and cheaper way to go than to regear the axles you have.

I won't get into the whole 8.8 swap stuff, but wait a second ... a bunch of guys are getting ready to tell you about it.

Edit to add: I see you have a 2.5" lift kit. Why are you not going with a larger tire? Like 31" maybe? Still great road manners and a good matchup for those 4.11 gears.

Hey bpounds,
Yup, The PO has 33x12.5's on it and a really worn out 2.5 lift, and with the ba-10. When I got it, the jeep wasn't driveable. Everything was wornout and I think the jeep sat for years as well. The 33's had a date code showing them to be really old, like 18 years old. They looked perfect too. But since the jeep drove so bad I swapped the out with stock wheels from a parts Jeep that I had.

I liked the look of the 33's but hated how they drove. I wish now I went to stock leafs as well. After all my work, the jeeps drives very well now. I call it my street jeep because the only dirt it has seen with me are a few dirt roads. I would off road it, it seems very capable but I really want it as a DD. I have a few other vehicles and its not like I have to drive this jeep but I love driving it everywhere, and do. The only issue is that it doesnt do highway with these gears and this motor. This motor gets angry if I hold 25-28k rpms on the highway for very long. Oil pressure drops real low. But its perfect at 60 or less, and I can drive in hot weather stop and go traffic for hours and the oil pressure stays fine.



Funny you mention the C clip thing. I picked up a pair of complete axles from market place for 150 bucks total. They have the 3:07's and I'm prepping them for install now. I am doing the rear axle this weekend. These do not have the C clip in them. I had the axles out to re-pack the bearings and to check out the seals. I though posi's were the ones with C clips, but maybe it was a year thing. I'm not sure if the 4:11 axle on my jeep have the C clip or not though.

So to conclude, next year I'll probably get rid of these springs and associated lift parts and then the jeep will be back to stock. It has been nutter'ed but still has a lot of the emissions stuff and vacuum lines and the carter carb on it still. I'll go with HEI and a non computer carb and clean all that crap off.

Thats just so my old *** can work on this anywhere and anytime, lol. Plus I think it will run even better, though it runs pretty darn good now. Never stalls, excels good, etc. Starts hard because no choke but once it starts, it fires right up the rest of the day instantly.
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post #5 of 15 Old 07-23-2021, 11:28 AM
Towny72
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How far from PA are you?

I would be happy to swap out my 3.07s for some 4.11s
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post #6 of 15 Old 07-23-2021, 01:57 PM
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Before mine turned into a garage ornament, it had 3.07s, ax-15 and and 33s, and I was genuinely stunned to find that it had those gears as it was so pleasant to drive both on and off road. I though I had at least 3.73s. I could rock crawl, and use 5th most of the time on the freeway. Granted, I have a 4.0, I don't know how much difference there really is power wise between the 4.0 and 4.2.

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post #7 of 15 Old 07-23-2021, 04:58 PM
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My 1990 came stock with 3:07, auto trans, 4.2 and was originally from Massachusetts. I thought most of the 5 speeds came with 3:73 since I have run across a bunch like that. Optional maybe or maybe a California thing?

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post #8 of 15 Old 07-24-2021, 06:02 AM Thread Starter
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Quote:
Originally Posted by calisk8er1080 View Post
Before mine turned into a garage ornament, it had 3.07s, ax-15 and and 33s, and I was genuinely stunned to find that it had those gears as it was so pleasant to drive both on and off road. I though I had at least 3.73s. I could rock crawl, and use 5th most of the time on the freeway. Granted, I have a 4.0, I don't know how much difference there really is power wise between the 4.0 and 4.2.

The biggy is the torque on these 4.2's and where it comes in. At 2k rpm's I'm at max torque which is only 7 less than the 4.0 but in way earlier. Its like a diesel engine to me. But that is good to know that even with 33's its ok.
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post #9 of 15 Old 07-24-2021, 06:05 AM Thread Starter
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The rear axle is in. I took it for a ride yesterday and like it so much more! Today I'll torque the leafs and then hit the highway but even around town and going 55 its revving so much less and is much nicer now.
On to the front axle shortly. That will be more involving. The rear was really easy to do.
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post #10 of 15 Old 07-24-2021, 06:17 AM Thread Starter
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Originally Posted by Towny72 View Post
How far from PA are you?

I would be happy to swap out my 3.07s for some 4.11s

I have axles already but once I do the fronts I'll sell these for sure. I'm 300 miles from Pittsburgh. I used to work there and drive home on the weekends...it was fun..
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post #11 of 15 Old 07-24-2021, 09:29 PM
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I had 3.07s when I bought my Jeep. I couldn't find anyone to take my axle so it went to the junkyard.

I thought it had 3.73 gears because, like you, I had smaller, lighter tires and it ran pretty decent on the freeway. Got up to 21mpg on the I-5 one time on a 600 mile trip. Going uphill or against the wind was impossible in 5th gear, so 4th got used a lot. When I went to heavier tires fifth gear was completely unusable and I hated the lack of power. The 4.10 gears helped.

Anyway, I see you already got the 3.07 axles so my point is moot. But if you don't do much hill driving they will work great.

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post #12 of 15 Old 07-24-2021, 10:26 PM
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Quote:
Originally Posted by jbolty View Post
My 1990 came stock with 3:07, auto trans, 4.2 and was originally from Massachusetts. I thought most of the 5 speeds came with 3:73 since I have run across a bunch like that. Optional maybe or maybe a California thing?
I always thought it was the opposite, as these things were built in a time when automatics weren't as mechanically good (or better these days) as a manual. For example, I always thought the only way to get 4.10s factory was a 2.5 with an auto, which would kind of back that theory up.

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post #13 of 15 Old 07-25-2021, 08:21 AM
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Quote:
Originally Posted by calisk8er1080 View Post
I always thought it was the opposite, as these things were built in a time when automatics weren't as mechanically good (or better these days) as a manual. For example, I always thought the only way to get 4.10s factory was a 2.5 with an auto, which would kind of back that theory up.
there always seems to be a twist with oem jeep parts but to my knowledge the 4cyl 5 speed all came with 4:10 gears, the only 4cyl auto I have seen personally had 3:55 but don't know if it was original.

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post #14 of 15 Old 07-25-2021, 10:17 AM Thread Starter
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Originally Posted by Awesome View Post
I had 3.07s when I bought my Jeep. I couldn't find anyone to take my axle so it went to the junkyard.

I thought it had 3.73 gears because, like you, I had smaller, lighter tires and it ran pretty decent on the freeway. Got up to 21mpg on the I-5 one time on a 600 mile trip. Going uphill or against the wind was impossible in 5th gear, so 4th got used a lot. When I went to heavier tires fifth gear was completely unusable and I hated the lack of power. The 4.10 gears helped.

Anyway, I see you already got the 3.07 axles so my point is moot. But if you don't do much hill driving they will work great.
Yup here in MI around Flint its flat farm land mostly. I see you have the 4.0. This 4.2 has no problem going 45 mph in 5th gear. I'm around 1500 rpm's and can still accelerate (slowly). I know most folks thing the 4.0 is a good motor, and it probably is but these 4.2's with peak torque at 2k are great in my book. The 4.0 peak torque is at 3k so yea, I could imagine 5th gear at 1500 rpm's being an issue for that motor especially with big tires.
I get a kick out of the size difference between the tires I had, and the tires I'm using. Those 33x12.5 were huge and heavy. It seems like they would play heck on the front end parts.
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post #15 of 15 Old 07-25-2021, 02:33 PM
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Originally Posted by yjcharlie View Post
I get a kick out of the size difference between the tires I had, and the tires I'm using. Those 33x12.5 were huge and heavy. It seems like they would play heck on the front end parts.
Yep. Pretty much why I went with 1/2 ton front end parts. I put those Goodyears on a Blazer I owned. I was impressed by how nicely they rode and they performed really good in snow. Way cheaper than most other options as well.

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