Sorry - another gear question... - JeepForum.com
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post #1 of 33 Old 07-17-2021, 06:27 PM Thread Starter
bicyclemonkey
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Sorry - another gear question...

Howdy - I'm having trouble deciding b/wn 3.73 and 4.10 gears. Fixin' to do a one piece axle swap since I have a leaky seal and may as well stick some more appropriate gears in there. Mine is an '84 CJ-7, 2.5" lift, 258, T4, 32" tires. 90% on-road, hilly country. The off-road portion would only be logging roads. I need to be able to run 65/70mph. Would the 3.73 be better with 32's and the 4-speed?

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post #2 of 33 Old 07-17-2021, 09:26 PM
Techlight
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You'll probably be happier with the 3.73 considering your usage and the T4...you can go here and compare the gear sets and their effect on your RPM's at different speeds.

http://www.grimmjeeper.com/gears.html

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post #3 of 33 Old 07-17-2021, 10:51 PM Thread Starter
bicyclemonkey
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Thank you..after looking at the grimmjeeper site, I'm also thinking the 3.73's would be the best bet.
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post #4 of 33 Old 07-18-2021, 07:23 AM
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I agree with Techlight. 410’s will be a little low for 65-70 mph, if that is truly what you want.

What gears are you currently running?

Why do I own a CJ? I like to “Balance the wrench and the steering wheel”
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post #5 of 33 Old 07-18-2021, 07:37 AM
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Gearing generally effects three things:
First gear (crawl)
Closes/opens the shift points
High gear (highway running)



What you have and what your intended usage is determines the best ratios for you.
Given your running a mild sized tire (32"), have a four speed (no OD), and you are looking at mild offroad use that doesn't require a low first gear, IMHO, 3.73's would give a nice boost in performance across the board.

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post #6 of 33 Old 07-18-2021, 02:32 PM Thread Starter
bicyclemonkey
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Quote:
Originally Posted by Axhammer View Post
I agree with Techlight. 410’s will be a little low for 65-70 mph, if that is truly what you want.

What gears are you currently running?
I'm not sure but according to the VIN it originally had a 5-speed. I'm going to find out tomorrow what gears are in it. It's currently at my Brothers dealership (he's the ops manager) for the axle seal repair. I'm going to have them count the teeth to get the ratio unless it's marked on the ring and pinion somewhere. Who knows, they may have been replaced at some point in the past.

If it still had the 5-speed, I'd just go with the 4.10 and put 33's on it when these tires wear out.
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post #7 of 33 Old 07-18-2021, 05:57 PM
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[QUOTE. If it still had the 5-speed, I'd just go with the 4.10 and put 33's on it when these tires wear out. [/QUOTE]

That would work good, but since you apparently have a 4 speed without overdrive, 373 might be best for you.

Why do I own a CJ? I like to “Balance the wrench and the steering wheel”
1985 CJ-7 Sebring Red, White hardtop, 284 CID inline six (4.7) TF 999 auto
2020 JLU Black, Black hardtop, EcoDiesel, 35’s on 2” Mopar lift
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post #8 of 33 Old 07-18-2021, 07:53 PM Thread Starter
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Quote:
Originally Posted by Axhammer View Post
[QUOTE. If it still had the 5-speed, I'd just go with the 4.10 and put 33's on it when these tires wear out.
That would work good, but since you apparently have a 4 speed without overdrive, 373 might be best for you.[/QUOTE]

Yes, I think you're right. I'll find out what gears I have in there now and go from there.
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post #9 of 33 Old 07-21-2021, 07:15 PM Thread Starter
bicyclemonkey
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Quote:
Originally Posted by Axhammer View Post
I agree with Techlight. 410’s will be a little low for 65-70 mph, if that is truly what you want.

What gears are you currently running?
It looks like it has 3:31 in there now. Is it even going to make much of a difference going from 3:31 to 3:73? Cruising at 65 would be more than adequate, I think.
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post #10 of 33 Old 07-21-2021, 11:24 PM
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3.31 to 3.73 is a 9% difference, quite a lot.

Peak power is around 2400 rpm, you need to be cruising slightly below this to avoid lugging the old girl.

3.73 = 61mph , a bit slow if you have aspirations of highway work, you will feel the engine is coming out of the hood at 65 at 2550rpm.
3.31 = 69mph , a bit faster than I would cruise in a CJ, dangerous in fact. At 65 it would be turning 2130rpm.

So if your aspiration is the highway, I would be tempted to stick with 3.31:1. Of course it depends on getting enough power from your 258 and its weight as to whether you can cruise at any speed. There are performance improvements which on a 1985 could be significant. What is the current on road performance like?

When you move to larger tyres, you will need to consider regearing.

BagusJeep lives in Bali with far too many 4x4s:
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
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post #11 of 33 Old 07-22-2021, 05:42 AM
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Quote:
Originally Posted by bicyclemonkey View Post
I need to be able to run 65/70mph. Would the 3.73 be better with 32's and the 4-speed?

The answer is yes.

I have 32" tires with 4.10 gears and "had" a T4 trans. With the T4 on the highway, it was a little high on rpm range so we opted for a T5 transmission instead. Now the Jeep's 258 engine is happier on the highway.

Around town in our very hilly area of PA I never use 5th gear and the 4.10 gears pull nicely. A lot of get up go but I can shift into 5th @ 50mph if I wanted to and it will still accelerate.


My CJ original came with 3.31 gears, only option available for the I6 engine due to the 80's fuel economy crisis, and it was a dog with 33" tires. It couldn't get out of its own way on the highway and forget about passing a semi on a hill. The wind resistance alone that comes off the front of a semi was enough to hold the CJ back from passing.
And you must also consider the weight of larger diameter tires too. Much more mass to turn so proper gearing is a must.

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post #12 of 33 Old 07-22-2021, 08:04 AM Thread Starter
bicyclemonkey
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OK, I've rethought the 65/70mph requirement. I can avoid the interstate and just take the back roads and side streets instead (more fun anyway). 60/65 is fast enough.
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post #13 of 33 Old 07-22-2021, 12:29 PM
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I have an 80 CJ-5 with 258, T-176 (3.52 low gear) D300 and ran 32's with 3.54 gears and though it was OK, I felt that a little lower gears would have made things "just right". I think 3.73's would go well with 32's.
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post #14 of 33 Old 07-24-2021, 10:06 AM Thread Starter
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So in keeping my Jeep off the interstate and sticking to roads with a 55MPH speed limit, so I'd be looking at a 60MPH top speed at most I would ever drive...and planning on 33x10.50 tires next go around, would 4.10's be OK, or should I still stick with 3.73?
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post #15 of 33 Old 07-24-2021, 10:24 AM
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Quote:
Originally Posted by bicyclemonkey View Post
So in keeping my Jeep off the interstate and sticking to roads with a 55MPH speed limit, so I'd be looking at a 60MPH top speed at most I would ever drive...and planning on 33x10.50 tires next go around, would 4.10's be OK, or should I still stick with 3.73?
3.73's.
I made the switch from 3.73 to 4.10 running 31's (looking for a lower first gear). Too low for open road driving (55MPH). I'm forcing myself to step up to 33's in an attempt to counter the added RPM's, since I don't want to regear. I wouldn't hesitate to run 33's with the 3.73's given your projected usage.
I've driven a number of rigs with 33"'s and 3.54 gearing and felt that setup was right on the edge of being too high.

Paperwork will ruin any military force.
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