AMC 20 Rear Gearing

[81CJ5baller]

I have an 81 CJ5 with an old 4 that i am pulling to swap in an 8. I know the SR4 is going but i want to keep the AMC20 rear and Dana 35 front. I want to say it is geared at 3.54 or maybe it is the 3.73. Cant remember. Anyhow, i contemplated rebuilding and putting a 4.10 in it or so. I am swapping the axles and welding the tubes and trussing but i am not sure on the gearing. The R&P equipment in the 20 is larger than the Dana and is actually quite solid. The tubes and the shafts are the downside but i plan to take care of that. If i have to rebuild the R&P, i will probably just look for a 44 and start over.

As man-ly as i would like to be, it will probably be a mostly street rig with some dirt but not anything radical. I dont want the 8 to be wrapping high RPM's one the freeway in order to maintain the 55-65 i will be running.

Question is will the 3.54/73 be the right gearing to keep the engine in the middle of its power band or should i up it? Any help is appreciated. And in sake of disclosure, i might sound like i know way more than i do....dont be fooled!!! Help is always welcome!!

Keep the top off and the Kenny Chesney on!

Ciao.

 

[hole]

What size tires are/will you be running? For mostly street/highway use, the stock gears should work just fine unless you're gonna run gigantic tires.

 

[mtnwhlr]

What size tires are/will you be running? For mostly street/highway use, the stock gears should work just fine unless you're gonna run gigantic tires.

 

[Jimbos76cj]

x2 on what size tire are you planning on running. with 3.55's to stay in the sweet spot range you will need to keep the tire size between 26' and 28" for 3.73 you can sneak up to 30's. and to change up to a 4.11's you would be good up to 33's.



Google Image Result for http://www.southwestpowersports.com/gear-chart.gif

you want to stay in the black in the chart for both good milage and power. These charts are designed to show RPM's at 60mph with a 1.1 final drive which is what your SR4 has.

Have you looked at every thing involved swapping in a 44? to find one already sized for your CJ will require one to be cut down of to get lucky enough to find one already done, then you will normally pay out the nose. if you decided to go full width waggy 44's you have to outboard the front and probably still have to have them rebuilt or gears changed. IF you lived closer I know a guy selling a AMC20 that already has a locker and one peice shafts for cheap around $450.

your next best option would be to look for a ford 8.8 rear. alot of them come with LSD and 4:10's from the junk yard, and have rear disc brakes. you will need a adapter for the rear to get it back to 5x5.5 bp.. I think.. and a matching thickness spacer for the front then obviously have the front
regeared.

If it were me Id pull the diff cover and see what gears you have in the axles now. alot of 4 cyl jeeps came stock with 4:10's at least the YJ's do.. Im not 100% on the CJ's. Dont go by the letters punched on the axle or the good old jack it up and spin the tires.. take the time and look at it.

Good luck

 

[S76]

I just went through this exersize on a CJ7 . I am installing a 5.0 and it will be a daily driver. I have a T177 so no Overdrive , and plan on running 33 inch tires.

That RPM chart is fine if you want your engine in the power band at highway speed. I don't. I want it throttled back and getting good gas mileage. Besides, with a V8 I have plenty of power the next gear down if I want it.

So for my application I could not decide between 3:54's or 3:73's. I happened to come across used 3;54's so that made the decision. At 65 MPH I should be turning 2350 RPM's. That's plenty.

And, the Jeep had 2:73's with 28/29 (?) inch tires, and I have to get a new speedometer gear because I am now geared 10% lower than it was.

So I think what you have is fine wether it is 3:54 or 3:73.

Just my $.02

Bob