'75 CJ5 gears

I'm thinking about re-gearing the differentials in my 1975 CJ5 when I
do the restoration. I'm fairly certain it has 4.27's in it, and I'd
like to go to 3.73's.

I see in my catalogs that there are two differential cases (the part
that the ring bolts to, that has the planetery gears in it) listed -
one for higher gears, one for lower.

I was under the impression that it required a whole new axle housing -
the main superstructure - to change from low to high ratios, and vise-
versa. Am I wrong? Does it only require the new ring/pinion and a new
diff. case?

Also, how much speed difference can I expect going from 4.27's to
3.73's?

Alex

 

Alex, you do realize that changing axle gears is likely the one automobile mechanic task that requires the most skill and knowledge? Very few amateur mechanics are up to the task. If you can't do it yourself, then it would cost at least a few hundred dollars per axle in labor, in addition to the parts. Yes, you need a different case, but the cost of the new case is not significant compared to the cost of the gears and the labor.

Why do you want to get rid of the 4.27s? The optional 4.27 ratio is MUCH more desirable than the 3.73s. I had a '73 with 4.27s and 31s, and I could easily run at 75-80-85 mph if I wanted too. Do you need to go faster than that?

If I were nearby, I'd happily swap my 3.73 axles for your 4.27s, and I'd do the swap for you, and pay for any new minor items needed. Certainly the right way to do this, if you (foolishly IMO) insisted on changing, would be to swap axles not gears. I'm sure that there is someone with 3.73 axles nearby that would swap for your 4.27s.

BTW - terminology - the largest axle part with the big casting in the middle and the tubes sticking out is usually called the housing, or more properly the carrier. The thing that the ring gear bolts to is popularly called the carrier but the more proper name is the case. You only need to change the thing that the ring gear bolts onto, inside the housing/carrier.

 

X2!!!!

If I were closer I would swap with you!!!

If it is the RPM at speed that is bothering you, I would suggest a easier alternative. Do a 2-3" lift with new leaf springs/shocks and run a 33" tall tire. That to me is the perfect size tire and gear combination for 3rd or 4th gear with a 1.00 to 1 ration. It will give you plenty of power off the line and lower your RPM at 60-65 mph.

Oh and it looks good too!

 

Thanks for the reply, Tim.

Yes, I realize it's an "advanced" job.

Why do I want to change? IMHO it seems that the Jeep is revving at 45 MPH what would feel comfortable to me at 55 or better. Maybe it's just my inexperience, but to me it feels uncomfortable at anything more than 40. It may not be turning as many RPM's as I think it is, though - I'm probably a poor judge.

Keep in mind I'm not keeping 31 inch tires - when it gets new tires/wheels they'll most likely be just a size or so bigger than what came as OE on it. Will a change from a 10.5 inch wide tire to a 7 or 8 inch wide tire reduce drag, and make it seem that it's not working as hard?

No, I don't need to go faster than about 65, really. A nice speed to be going when it's turning 3000 RPM would probably be 60 or so - am I dreaming?

Was yours a 304?

I'm not absoultely 100% sure it has 4.27's - I haven't counted teeth. The tag on the front differential has "4.11" stamped in it, among other numbers. When I have it apart I'll count teeth to be 100% sure what the ratio is.

Why are 4.27's so much more desirable?

 

The 4.27 gear ratio works better with taller tires like the 33". Also you start off a little better at stop lights. I have the 3.73 and with a 33" tall it takes more power from the engine to get going from stop. Off road the 4.27 gears work better because you can go slower on obstacles and manage throttle and steering more effectivly. The transfer case D20 has a high 2.08/1 gear ratio so off road with 3.73 is quick and not as controlable as with lower gears.

If you are going to a smaller tire than 31" then the swap to a 3.73 may have some benifit on road but off road would still suffer.

 

Overdrive - is there such thing for a 70's CJ?

Thanks for the explanation, coby61.

 

http://www.chrismans.com/Calculators.html

Check out the "Find MPH" and "Find RPM". This will help you to determine what gear ratio is needed with what size tire you plan to run. 3rd gear in a 3spd trans and 4th gear in 4spd are usually a 1 to 1 ratio and the transfer case in high is 1 to 1 ratio also.

 

Yes, my '73 was a 304. Shouldn't really matter though - a 258 and a 304 can both turn plenty fast enough for those speeds. If you have a fixed fan (no fan clutch) that will be louder than the thermo clutch fan on the 304.

If it's 4.11s, those are not available on the CJ from the factory. If they are, they are between the 3.73s and 4.27s in engine speed.

The 4.27s are desirable just because of the lower (higher numerical) ratio. Everybody wants better gearing, and that's one way to get it. A 4.27 ratio is a pretty good compromise if you have taller tires ...

There are several gear ratio calculators on the net. Hre's one http://www.novak-adapt.com/knowledge/gearing.htm - if you have 4.27s, 31" tall tires will give you 65 mph at 3000 rpm.

 

From what you guys have said and from those calculators, it appears I'm a very poor judge of engine speed! Now that I've seen the light it looks like I don't really need faster gears. I don't intend to drive mine on the interstate very much, so most of my driving will be at 55-60 MPH. And if the change in speed is noticable while crawling also, I guess it's a good compromise.

Thanks!

 

TY for posting this nice little tool, helped me some too. With my setup I will still be at about 3000rpm with my 33's and still around 2500 rpm if I move up to 35 or 36's.

Alex,,just buy a tach,,much cheaper than a gear swap and you can accurately monitor your vehicle vs engine speed.

 

Take a second look t the tag on the axle. Are you sure it doesn't read 41-11 which would be 3.73 gears (41 ring gear teeth divided by 11 pinion gear teeth =3.727=3.73)? But this doesn't rule out the possibility that some one may have changed the gears in the 33 years that the Jeep has been around so when in doubt, pull off the diff cover and count teeth.

 

I would gladly have 4.27 gears. I have 34 inch tires and according to the tags I have 3.08's. I don't think they are this tall but without opening the covers I won't ever know. 4.27's are better in the woods and on the street because they have better low end. Just my thought. :stout:

 

Try the website "grimmjeeper.com".
Fill in the info, punch the buttons.
Will give you all the answers.
The secret to success is being able to work within the power/torque band of the engine.
I have the info from a '73 data book. Don't know whether this info is available anywhere else for current engines.

 

Thanks, guys. "80cj" - I'll look at the tag and see if it is "41-11" and not 4.11.

 

I looked on the tag on the front diff. - on the top line, none of the numbers seem to make any sense, but on the second line it says "373 41 11" - 3.73's? I don't know why I didn't see the "373" and thought the "41 11" was "4.11" but it appears, if nobody has changed gears since it was new, that my CJ5 has 3.73's. Thanks for helping me solve this, guys! You're great!

 

Great. 72-75 CJ's never came with 4.11 gears so when you said "411" I suspected you were reading the gear tooth count incorrectly. Those gear ratio tags get pretty cruddy over time and some are darn near impossible to read. The top row of numbers is the Dana axle part number ... I think. Now as you say, hopefully, no one has changed the ratios. If you've never changed the oil in the diffs, it wouldn't hurt to do it and at the same time you could verify the gear ratio. Dana stamps the ratio on the edge of the ring gear or you can also count ring and pinion gear teeth.

 

I swapped a set of axles I pulled out of a '73 CJ5 into my '83 CJ5. The tag on the front axle siad it was 3.73's and the front axle WAS 3.73's, however the rear axle had 4.27's. I swapped a set of 3.73's into the rear and kick myself for not changing the front to 4.27's.

 

I'll have both diff. covers off later in the restoration - I'll be changing every fluid in the Jeep before it's put back on the road. I'm big on maintainence :twisted: