Gearing Advice Needed

Gearing Advice Needed

Coach

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Location
Dallas, TX
Vehicle(s)
1986 CJ7 / 4.2 I-6 / Auto / Dana 30 / Dana 300 / AMC 20 / Hard Top
Since my CJ just went over 100,000 miles on its 258, I'm thinking of delaying the engine swap a while. So now I'm looking at my gearing as a way to upgrade for the future while helping my current engine and drivability at the same time.

My question is what gearing is going to be the best bet. I have 31's now, but plan to go up to 33's when they need to be replaced. I'm running a 258 with a Motorcraft 2100 carb, with an automatic transmission. What has me the most confused is what I'm finding about how automatics change what gears you would want to run. I plan to stay with an automatic, even after I swap engines down the road.

I drive 90% of the time in town and on roads that are 55-60mph. The offroading I do is more fireroads and light crawling, with no mudding. I would like to keep my gas mileage at 14mpg+.

Any advice or tips would be greatly appreciated.
 
I would say you should have gear ratio of about 4.11 to 4.56.
_35422.jpg

The above chart will help decide on a gear ratio. It shows the RPMs to expect at 65 MPH. Anything in green is a good compromise between power and gas mileage.

Also check out derf's web site for a cool Gear Ratio Calculator http://www.grimmjeeper.com/gears.html
 
Last edited:
I would say you should have gear ratio of about 4.11 to 4.56.
_35422.jpg

The above chart will help decide on a gear ratio. It shows the RPMs to expect at 65 MPH. Anything in green is a good compromise between power and gas mileage.

Also check out derf's web site for a cool Gear Ratio Calculator Gear Ratio Calculator

Looking at the chart and not having checked myself, it looks like I'm almost off the bottom of the chart for 31's. My rpms barely get above 2100 at 65.

If I go with 4.56, do you think I would see a big improvement in my drivability around town?

Appreciate the help.

Coach
 
Looks like you have the factory 2.73 ratio.


Wide Track AMC20
Code Gear Ratio Diff Type
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
 
Coach, I have the 3.54 gearing that came in mine and I would not change it for my use. I had 31's and all was good then went to 32's and still good for allaround use, my CJ is not a rock crawler or dedicated trail rig but one that tries to cover the bases by being able to keep up with traffic and still be ok on the trail. I do have a T-18 though and that makes it more versital in terms of gearing.
Just another viewpoint :D.
 

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