more on gears

[AKASHAJEEP]

ok i have made up my mind and am going to go through with 33X12.5 on a true 5.5" lift. now gearing comes in to play with bigger heavier tires.
I have a 96' auto with the 4.0 H.O. and the towing package. but dont know what gearing is in there.
right now i am runing 30X9.5" bfg.s on 15X8 cragar rimz. everywhere i look the charts say that with 30" tires and 3.55 gears my rpms are at 2490 and 2584 rpm for the 3.73 stock gears. im not really sure what exactly are in my 8.25" but thats not exactly what i am noticing.
when the gear ratio is higher it actually raises the rpm of the motor. i guess i am really confused here because i thought that when you gear higher the rpms would go down on the motor if you are running the same size tire?
when I am running at 65 mph on the highway my rpm's are at 2000 dead even. and 2100-2150@70mph.
now i am thinking of going with a set of 4.56 gears cause that is what everybody says to go with but i do not want to sacrafice my motor for turning some bigger tires if you know what i mean. I really dont want to loose any performance or much in terms of mpg then what i have already with the lift kit. (not exactly the best thing for mpg)(brick in the wind)
argg im really confused here and could deffinatly use some hlep.

 

[graewulf]

if you want the factory specs - email jeep (there is a link on their site) with your VIN and they will email you the factory options list. This will tell you things like gear ratio, etc.

 

[bchcky]

RE: towing capacity of a ZJ

at the off chance something has been changed, there is a plate on the back of your diff's, read the numbers off those, and then inquire with those numbers, not the VIN, those numbers on there tell you exactly what axle you have, what the gearing ratio is, all that good stuff.

 

[TwistedCU]

RE: what size can I run?

My 8.25 has a tag on the end of the axle next to the drum. Mine said:

3:55 open

Meaning 3:55 gears, open differential.

 

[AKASHAJEEP]

RE: Read Disks

well I got some time today and went out to take a look for the plates and well couldnt find jack anywhere except on the housing itself. and the numbers are made in the cast mold along with the diff. one tag though that i did find is a small square on one of the bolts that holds the diff cover on and it says 35.

btw these are the numbers that i found on the housing. not sure how to go about decoding these or if they will give the info I need?

these are on the bottom side closest to the cover.
5H21E2L2
MS 3760 A
these are on the bottom side closest to the drive shaft
26670
4384213

 

[AKASHAJEEP]

 

[Bounty__Hunter]

Should be 3.55 gears, but check for a metal tag on the front axle to be sure. Should be bolted between 2 diff cover bolts, first three numbers of the 2nd line on the tag.

Can you post a pic of the front of the rear diff, opposite the diff cover?

 

[AKASHAJEEP]

RE: need space for tires, what

ya i will later on tonight. have classes right now, but i will get the pic loaded at least for now!!

 

[TwistedCU]

Hey Bounty, any idea what year they started tagging the axle tube (like on my 2000)?

 

[AKASHAJEEP]

well here is the front diff and i didnt see any tags on it at all. but I will get back under there tommorrow and clean the edges up some, maybe it is just covered with stuff?

 

[AKASHAJEEP]

Can you post a pic of the front of the rear diff, opposite the diff cover?

hmmm, i guess i misread that the first time around. ill get one up tommorrow soemtime.

 

[AKASHAJEEP]

theres the pic ya wanted.

and heres the one showing the tag (FINALLY) I did have to take a wire brush to it in order to read it but, 3.55 are what it has!!

 

[AKASHAJEEP]

ok now that it is settled that I do in fact have 3.55 gears in there now, when i step up to a 33X12.5 would it be better to run the 4.10 or 4.56? I would like to keep the performance about where it is right now and a little better launch wouldnt be a bad thing as long as the rpm's at higher speeds dont suffer too badly. as I said before at 65 mph I am running at 2000rpm, and 2150 @70mph. I do not want to see my motor reving any higher than 2300 rpm at 70mph when I do this swap. and I cant figure out these gear ratio graphs for the life of me.

 

[Bounty__Hunter]

So do you know for sure if it's a 35 or 8.25 in the rear? I'd have to see a pic of the diff housing right above the pinion yoke to be sure.

I'd got with 4.56, it'll give you the performance boost and help make up for the added rotational mass as well as the larger tire.

 

[AKASHAJEEP]

im about 95% sure that the rear is a 8.25, because there are a few chrysler markings on the diff and nothing with a dana logo on it like the front has, also the cover is exact 8.25" charistics. with the lip and plastic fill plug thingy.

back on topic here, do you think that by using some pro comp Xterrain 33's would be too much for a set of 4.10 gears? I really dont plan on going any bigger than a 33" tire at all. and if I do then other mods would be on the way to step up to some 35-37" boggers.

I treid my hand at some conversion with the charts and found that there is roughly a 22.5% difference in rpm vs tires size stated on the charts, and according to them the 4.10 would be best in "closets to stock" performance. but again I would like to have a better launch off the lines, (lower crawl ratio) ect. but dont want to sacrafice gas milage too much for that. she dosent have any problems smoking the rears if needed as it is right now. (I just need to work on getting the exhaust sound right)

sorry I am being so finicky with this topic but akasha is my daily driver that sees more than 400 miles a week. I dont want this change to affect the mpg ratings (that already suck enough) too much along with doing more damages to the motor by reving higher than needed.
hope you guyz can understand the situation here. and as always your help is greatly appreciated!!

 

[IcyFridge]

I would go with 4.10 for 31-32" tires. 4.56 for 33 to 35" tires.

I ran 33's with my stock 3.55 on D35 on my 95. It was not bad at all. Still quick. However - I would go for 4.10s on 33's for a Daily Driver.

 

[Bounty__Hunter]

The gear calculator I used recommended a new ratio of 4.18 or around there. These gear calculators only account for the increase in tire height, not taking into account the large increase in rotating mass that a larger tire adds. For this reason I always error on the side of going a little lower with the gear ratio, like 4.56.

Another reason, 3.55 to 4.11 is not much of a change, and not worth the time and money involved IMO. Again I would go with 4.56 to get your moneys worth, more bang for the buck.

 

[mingez]

My RPM's are not excessive with my 4.56 gears and 33s. Just drove up from El Paso in my XJ today, and very road worthy.

How often do you drive on the freeway? I'll check today what the RPM's read on my XJ at 65mph so you can compare.

 

[AKASHAJEEP]

I use highways around here but speed limit is 65 and i usually set the cruise @70mph.
as for how often well I have been known to put more than 400 miles per week on her on the highway so, maybe 50/50.
I am getting like just over 18 mpg on the highway now and 15 or so (depending on how light my foot feels that day) in city driving conditions. sucks hardcore man!! and the further i have to put my foot in it the more it sucks. usually i have to pass at least 2-4 cars each time during that weekly run and i know i shouldnt be but i cant stand to be behind someone that is doing 55 in a 65.so fo course passing gear comes in and down goes the gas guage!! LOL

 

[mud4feet]

RE: Re: RE: Re: RE: Old Jeeps are better!!

............. but i cant stand to be behind someone that is doing 55 in a 65..............LOL

That's not you behind me on the upgrades on the way home from work, is it?? :lol: :lol: :lol: Sorry, with my little 4banger - that's the best I can do!! :lol: :lol: :lol: