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I have a 46 cj2a with a buick 225 v6. From what I can tell, it has a SM420 4 speed manual trans with dana 25 front and dana 41 rear axles with 33" BFG's. I do some mild offroading in it but I want to make it a little more streetable. Right now, it screams at me driving 45 mph. I don't even use first gear because it's ridiculously low. I'm not sure how to figure out what gears I have in the axles but I'm curious about which gearing would let me do 50-55 mph without the thing screaming at me. Is there an easy way to figure out my gearing? I assume the axles are stock. I just bought it a few months ago and the previous owner doesn't know anything about it, he bought it as I currently have it. Any help would be appreciated. I'm in the military so I am on a budget until Congress approves bigger pay raises for us. My mechanical skills are very limited but I'm hoping to improve them with this rig.
 

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I have a 46 cj2a with a buick 225 v6. From what I can tell, it has a SM420 4 speed manual trans with dana 25 front and dana 41 rear axles with 33" BFG's. I do some mild offroading in it but I want to make it a little more streetable. Right now, it screams at me driving 45 mph. I don't even use first gear because it's ridiculously low. I'm not sure how to figure out what gears I have in the axles but I'm curious about which gearing would let me do 50-55 mph without the thing screaming at me. Is there an easy way to figure out my gearing? I assume the axles are stock. I just bought it a few months ago and the previous owner doesn't know anything about it, he bought it as I currently have it. Any help would be appreciated. I'm in the military so I am on a budget until Congress approves bigger pay raises for us. My mechanical skills are very limited but I'm hoping to improve them with this rig.
I say DO NOT change the axle gears. Leave them alone and get a Saturn
Overdrive from Advance Adapters. It fits into your D18 transfercase.
It's cheaper than regearing both axles, plus it keeps those low gears for
when you're rock crawling. I had the same motor/trans/t-case set up
in my 1970 CJ5 with the overdrive. Best of both worlds.

https://eshop.advanceadapters.com/commerce/ccc1105-overdrive-units.htm
 

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I say DO NOT change the axle gears. Leave them alone and get a Saturn
Overdrive from Advance Adapters. It fits into your D18 transfercase.
It's cheaper than regearing both axles, plus it keeps those low gears for
when you're rock crawling. I had the same motor/trans/t-case set up
in my 1970 CJ5 with the overdrive. Best of both worlds.

https://eshop.advanceadapters.com/commerce/ccc1105-overdrive-units.htm
X2. I love mine. You can get the saturns new, or the usual routes (ebay, parts meet, etc) or you might try hermtheoverdriveguy. He often has reman units for sale. He rebuilt mine and it works great.

Your jeep most likely has 5.38:1 gearing. To check it, jack up only ONE tire. Put it in neutral. Spin the tire and count the drive shaft rotations. The math is driveshaft turns *2/tire rotations.

Example, spin the tire 4 times... say the driveshaft spins almost 11 times. 11*2/4 = 5.5, or close enough to 5.38:1. If you had a spool, you'd jack up both tires and it would just be 11/4, but an open differential has a 2:1 effect if the other tire is stationary..

If you really want different gearing, I'd suggest swapping to later model CJ-5/7 or wagoneer axles. You'll pick up strength, width, better brakes, and you'll also get taller gearing (borderline too tall). If you stay pre 73, you can get small strength increases and taller gearing with no width change.

I don't have a link handy but there are a zillion gear ratio-speed-tire calculators on the net. I've got one I made that I can send you but I think I'm the only one who can operate and decipher it.... does acceleration plots, top speed the whole works.... By my calcs, my flatty should hit 60 wayyyy after the 1/4 mile mark. And it's right ;)
 

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Discussion Starter #4
This is probably a stupid question but, how do I know which one would work for my set up? Keep in mind that I'm very limited on the mechanical side. It talks about tooth drive gear and spline output.
 

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This is probably a stupid question but, how do I know which one would work for my set up? Keep in mind that I'm very limited on the mechanical side. It talks about tooth drive gear and spline output.
There are application charts on AA's website that will tell you what you should have, but... you need to pull off the 5 bolt PTO cover in the center/back of the t-case and take a peak to know for sure.



The gear looks like this: You are counting the teeth on the outside and the splines on the inside (which are partially obscured in this picture by a washer). Ignore the little external teeth that are for a PTO drive. I think you'll need to pull off the nut and washer to count the splines on the shaft.

You might dig up the AA chart first, because I think certain gear tooth counts only came with one spline count.
 

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Discussion Starter #6
Thanks for the info. I'll mess with the PTO and the gearing math this weekend this weekend. It seems like it would be a pretty easy install if it deals with the PTO.
 

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im pretty sure it was the jeep trucks and wagons that had the different tooth count than the CJ models. i want to say that the trucks and wagon had a 29 tooth count and the CJ's where 27. thats why a OEM overdive from a truck or wagon wont fit into a CJ unless you swap the hole transfercase. agin this just is kinda shooting from the hip. i would search novacks or AA site, jason.
 

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dont think anyone answered your original question.....
lift up the back end. put a mark on your tire and a mark on one u-joint cap on the rear driveline. turn the rear wheel (now jacked up and free) slowly and count how many times the driveline turns during ONE turn of the tire. when the mark on the tire hits its original position, stop, and remember how many times the driveline turned ((ie....one tire turn for 4 and a half driveline revolutions is 4:56, one tire turn for 3 and three quarters turns of the driveline is 3:73 etc.....))
you get the point
i had a 47 2a built up pretty good. it did have a corp 20 and dana 30 though (unfortunately). it had a 350 and would do 85 mph's with 3:73's no problem. i hated those gears, even with granny low they were worthless on the trail, having to gun it to turn the tires in a tight spot led to 5 REAR DRIVELINE SHATTERS!....and they had CV's!!
take it from me, enjoy the fact that you have low gears, its better than the alternative.
 

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Discussion Starter #9
Yeah. I think I'm going to go with the Saturn overdrive unit. It seems like the best way to go and pretty easy install even for someone with limited mechanical skills like myself. I appreciate the info from you guys. I'll keep you posted when I get it going. It won't be right away because I have a few other expenses that I have to take care of first. I don't want to pull the money from the savings so I have to save up for it.
 
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