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Discussion Starter · #1 ·
I posted this in the tire and wheel forum as well, but thought it might get more traction here.

I’m new here, guys, so go easy on me. I’ve been doing research on this so long my head is spinning. Let me set the stage for you. I have a 1983 CJ7. It’s been retrofitted with a 5.0 v8 with a 4 barrel carb. It’s got a 4.11 differential and the axles tubes and diff housing have been gusseted. Best I can tell by measuring, it’s got a 4” suspension lift and 2” body lift. It does not have a SOA lift. Currently it’s got 15” wheels and old school Q78 gateway buckshot mudders. These are supposed to be 36” tires, but my best measurements show them to only be 34”.
The wheels are rusty and trashed out and I’m wanting to replace them with 17s. I want to go with a 35” tire at a minimum and maybe a 37” if it will work. I know at a minimum I should upgrade the diff to 4.56, and maybe even 4.88 with the 37s. With my lift, can I fit the 37s with no rubbing and no fender trimming? I do not want to cut any metal. I don’t wheel the jeep hard at all and it’s not a daily driver. I just use it here on the ranch. Terrain is flat to rolling, and it’s either dry and hard as a rock or wet sticky gumbo mud. I know this question has been asked a ton, I just want to make sure I get it right the first time. Thanks in advance for any help y’all can provide.
 

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I posted this in the tire and wheel forum as well, but thought it might get more traction here.
With a great opening line like that, I'll chime in by complementing it, nice job!

Advertised tire size and the actual size are often slightly different. A 33 x 10.50 15 R will normally be about 32.5" tall, give or take, depending on manufacturer.

When shopping tires, after you have selected the height & diameter, you should look at other dimensions and ratings as well. Load range (ply rating), weight, speed rating, among others should be considered.

I'll post the link for a website that you can use to see the actual tire height vice the advertised height, among other useful info. This link will also show the different makes/models tires available in the particular size you choose. Some sizes have a lot to choose from, and other sizes are limited, or don't exist. It's a great website for tire shopping.

Concerning gear ratio suggestions, what transmission you are running?

In closing, if you can post some pictures of your Jeep, that will help. There are folks on this forum will be able to evaluate your current lift height pretty close, just by looking a some quality pictures from different angles.

https://tiresize.com/tires/BFGoodrich/All-Terrain-TA-KO2-33X10.50R15.htm
 

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Discussion Starter · #3 ·
Sorry, I thought I put my transmission down. It’s a 5 speed manual. I’m not sure which specific one though. I’ll try and get some pictures posted. Any particular areas I need to focus on on?
 

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Discussion Starter · #4 ·
Also, the rear axle appears to be AMC 20 and the front axle appears to be a Dana 30, if that matters for anything.
 

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Doesnt the top of that front shackle hit the frame every-time you flex the spring and keep it from moving?
 

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You have stock axles but the AMC20 is gussetted. As long as it has single piece axle shafts it can handle up to a 37" tyre, if not then 35" is already putting it under a lot of stress. You have centre caps on the rear wheels so hard to see if there is an Axle Nut underneath, the hallmark of a 2 piece axle shaft.

4.56s are the lowest common ratio between the Dana 30 and AMC 20, you cannot go lower. At these extreme gears I worry that you are going to damage the teeth on the Ring and Pinion.

The transmission is important, the only 5 speed in 1984 was the T5. Fine in a Mustang but it is a passenger car transmission and quite long, the stresses of a built rig off road can twist it and cause it damage.

You do not say but I presume you have a Dana 300 transfer case, a fine piece of kit for 1984.

The 1st gear on a t5 is 4.03. The other gears are reasonably spread and there is an overdrive 5th of 0.86.

You have 4.11:1 gears and plenty of power.

The following chart is for cruising on highway at 1:1 ratio. You need to decuct 14% on the engine speed for having overdrive. On 35" tyres this brings you to 1861rm and on 37" 1724rpm. On highway with your current ratios and a 5.0, you are going to lug on 37" tyres in 5th, probably OK on 35".

If you went to 37" tyres and changed the diff to 4.56:1 you will be at 1,950rpm on 37" tyres, a trifle low but the torque of that 5.0 should make it driveable. It will not be a sports car but you have 4th gear for that.

Around the farm you have other issues, namely is the ratio in 1st low enough? I have 3.52 and 6.32 gears in Jeeps and the 6.32 is only good for climbing out of muddy holes with a full load. I think you should be OK with 1st.
 

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4.56s are the lowest common ratio between the Dana 30 and AMC 20, you cannot go lower. At these extreme gears I worry that you are going to damage the teeth on the Ring and Pinion.
If this is true, 456 is the lowest gear ratio available for your D-30/AMC-20, I would not consider the swap. The difference between 411 and 456 is minimal, so to me it's not worth the time and cost to swap gearsets. I would suggest running tires that suit the 411 gears you already have.

It sounds like your transmission is your weak link, but since you don't wheel it hard it may be OK.

As far as larger tires, it doesn't look like you really have much more room to go that direction.

You didn't talk about how it drives now, or how long you have driven the Jeep as it is now. I'm just trying to get a feel for how it performs with the current V-8, with those 34" tires and 411 gears. Is the gearing adequate as it is now?

I think your tires are plenty tall, for the lift you have now. Unless you want to wheel it hard, I don't see the need to go larger, but that's my opinion, and we all know what they say about opinions.
 

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Discussion Starter · #9 ·
I’ve driven it like this for the last 3-4 years. The gearing seems to be ok. It’s a bit of a dog when I’m in overdrive, but a quick shift down to 4th gear puts plenty of pep in its step. Upon further research, over half of my tread on the current tires is worn away, so they were probably pretty close to 35-36” when new. I think you are correct that these tires are probably as tall as my current lift warrants, any taller, and I’ll be trimming fenders. I’m going to look at 35-36” tires as a replacement for now. I’m currently waiting on my new lift kit to come in. My current springs are completely wore out and becoming flat. Once I get the new lift installed, I’ll see where that puts me as far as my true lift height and go from there.

On a side note, I found 4.88 gears for both the Dana 30 and AMC 20, so I’m not sure where Bagusjeep got his info. I’m not saying he’s wrong, just a quick Google search found the necessary gearing for both axles. Maybe it’s something new available since the last time he looked?
 

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I take it back about the 4.56:1 gears.

They were the lowest gears from the factory but in more recent years I see someone has started making them in 4.88 as well.

Well, you find out something new if you stay on the Forum long enough.
 
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