Advice on ratios 76 CJ7 AMC 20

[LtBest]

What ratio am I aiming for in my diff??

After reading and reading I still can't quite get my mind around what I need for good (read: sucks less than) gas mileage. I'm working on making a 'go to the beach / road trip' CJ. Definitely not a rock crawler. Maybe some trails but for now they're too far away with my AMC 258 i6 / 4.2l and T-150 .

I want:
T-5 (well aware it sucks for heavy off road)
Dana 300
AMC20
31" wheels.

I'd like to do the single piece shaft upgrade but I'd like to postpone UNLESS the mystery ratio requires the change out.

 

[mtnwhlr]

I want:
T-5 (well aware it sucks for heavy off road)
Dana 300
AMC20
31" wheels.
/QUOTE]
This the setup I have and with 3.23 or 3.32 I don't remember witch 5th gear is useless unless you are going down hill on the highway. I think a 4.10 would work the best, it would make it performe better around town and you would have 5th gear on the highway. If I was going to stay with 31s that is the way I would go but I would like to go to 33s or 35s so I'm thinking about 4.56 or 4.88 so it will crawl better off road.

 

[csuengr]

3.73's would work well.

 

[Old Dog]

What ratio are you running now and what don't you like about it?
Might help us determan what you need.
Cj's aren't known for their gas mileage.

 

[GatorBate]

Something else to keep in mind is that higher gear ratios between 2.73 to 3.54 takes a different carriage than the 3.73 and lower ratios. So if you have the higher ratios (2.73 - 3.54) and you want to run 3.73 or lower you will have to replace the carriage also.

 

[LtBest]

I plan on opening up the diff to see what I have. There isn't anything I don't like right now, I'm trying to marry up the gears as best as possible for hwy use on trips.

Maybe I don't know what I'm asking but gas mileage is affected by the gear ratio and wheel size etc. Power for crawling is also affected. So another way to ask is this: Less crawl, more highway = ??

. . . .not that I know how to actually change any of this dark magic that happens inside the mysterious ball of differential. . . that post is later

 

[norville]

There should be a stamp on the housing with 2 letters that can be decoded. Or just jack up the rear and count the rev's of the drive shaft vs the wheel.

 

[norville]

here's a link I found
How to Calculate Rear Gear Ratio | eHow.com

and a video

http://www.youtube.com/watch?v=2y9iqvO7RaU

 

[CJ]

Don't over look our technical posts... -->> http://www.jeep-cj.com/forums/f99/how-determine-your-gear-ratio-10217/

 

[norville]

don't over look our technical posts... -->> http://www.jeep-cj.com/forums/f99/how-determine-your-gear-ratio-10217/

excellent!

 

[LtBest]

Maybe I'm just wishin' -- wishin' upon a star!! (Brian Regan joke) but that imprint on the attached photo looks like a "3.73"

Advice por favor.

 

[Old Dog]

I'm not 100% but I think the numbers you want are the 11 and 39 (I think thats what I see) which would be 3:54s. I'd count teeth just to be sue.

 

[LtBest]

Yep. 39 Ring teeth and 11 pinion teeth. 3.54

The $million question:

Do I need to to any changes to the diff if I plan on Chevy 350, T-5 , Dana 300 for highway use, street queen, flat lander edition CJ7 ??

It 'seems' this is a great diff option since its A) free and B) close to the 3.73 advice earlier.

 

[mtnwhlr]

With a 350 it should be fine. Get every thing else done and see how you like it.

 

[Old Dog]

Right, the 350 should have plenty of power to pull the 3:54's.
I might rethink the T-5 though, maybe time for an NV4500.

 

[73CJ]

Gas mileage just plain sucks when you are pushing a brick wall down the road.

Gears will help, but don't expect 30+. . . .

 

[LtBest]

I thank you all for your advice. Being an A/V guy, this car gear business is something that is completely foreign to me. With the forum's help I now understand the why's and how of what's happening in the drivetrain and I can't thank you enough.