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Axle Gears

99_XJ_

99_XJ_

NAXJA Forum User
Location
Howard, Ohio
Have a '99 XJ (specs at bottom). With the larger tires Ive experienced some rather horrible gas mileage issues... were talking about like 8-10 MPG in the city... I had a cracked header but had it welded and replaced the O2 sensors...

I'm thinking maybe different axle gears to remedy this issue?
If that's the better route to go, how difficult is it to change said gears?
I would take it to a shop but seeing as I am but a poor college kid :cheers: it may serve me to do it myself. Ive worked on this thing left and right and i installed everything myself (except electrical). Also what ratio should i use? it has stock 3.55 gears.. i'm thinking maybe 3.73 or whatever is close to that?

Just thought I would ask. Thanks guys
 
I personally wouldn't go through the hassle of installing deeper gears if you plan on keeping 31s, but that's just me.

Gears aren't the simplest of tasks, as a fellow college kid I'm hesitant too if its something I want to tackle by myself or pay to have done right.

Read this and see how confident you feel.
http://www.pirate4x4.com/tech/billavista/Gear_Setup/
 
For what you would pay for gears you could have an axle with disc brakes and LSD. You can get an 8.8 with 3.73 and I believe grand Cherokees with the larger motor have 3.73 and some have LSD also. You can grab said axles from junk yards for around 150-250 and installation is 100% easier than gears
 
OK screw that mess... looks like its horrible...

About the axle... Would that fit? and what would I do about the front gears?
 
Also.. Would a new header solve the issue? Maybe the welds popped? Or a crack was missed? Just anything at this point to get better fuel economy... I know people with almost the same setup as I and they get MUCH better gas mileage...
 
Go to a pull-a-part and find an 8.8 rear from an Explorer. They come in 3.73 and 4.10. Then search the yard for an XJ with a front axle with corresponding gears.

Depending on the yard the axles could run between $75 and $150 each (mine were $75 each). Pay someone $100 to set up the 8.8 (spring perches and shock mounts need to be welded on), then pop them in and away you go.

That is the process I did and I am running 31's on a 2001 XJ. I was into the whole project for under $500, much cheaper than having the OEM set up re-geared.
 
Don't bother with 3.73 gears; they're not deep enough.

Stock tyre size is 28"; you've gone to 31". That's a (roughly) 10.7% difference. Adding 10.7% to your stock 3.55:1 gears, we get 3.92:1. Nobody makes a 3.92:1 gearset, so 4.10:1 is your next jump up and what you want to aim for. You'll be about 5% overgeared at that number, but it's not enough that it makes any practical difference.

Definitely re-check the manifold and gaskets. May be time to bite the bullet and just replace both.
 
Cottontail said:
Go to a pull-a-part and find an 8.8 rear from an Explorer. They come in 3.73 and 4.10. Then search the yard for an XJ with a front axle with corresponding gears.

Depending on the yard the axles could run between $75 and $150 each (mine were $75 each). Pay someone $100 to set up the 8.8 (spring perches and shock mounts need to be welded on), then pop them in and away you go.

That is the process I did and I am running 31's on a 2001 XJ. I was into the whole project for under $500, much cheaper than having the OEM set up re-geared.
Click to expand...
X2.

I'd aim for 4.10, they are about the deepest gears you can scrounge off a common vehicle in the junkyard. However, if you really want to do 3.73s, you can get a d30 out of a 93-98 ZJ with the V8 (iirc) which came with 3.73s sometimes. Make sure to check the tag first, and also verify that the pinion has a u-joint yoke on it instead of the CV joint cup they used on some ZJs. I believe some are low pinion and some are high pinion but I'm not sure. You may have to adapt to cam adjuster style LCA bolts.

If you find a 4cyl XJ with a 5 speed manual in the yard, check the frontend, it probably has 4.10s. They aren't that common around here though.
 
I know of a guy that says he knows of a junkyard that's like heaven for jeeps somewhere a tad north of here.. I will see what I can dig up... About the exhaust, the gasket is new but im funny about gaskets... I'm thinking just a cheapo rugged ridge header would do the trick.. and maybe replace the glasspack tube to a respectable muffler like a flowmaster 40 or super 44 or a dynomax or whatever its called..
 
Start with the basics, perform a complete tune-up including the coil. Replace the Coolant Temperature Sensor. Replace the thermostat. Fix the exhaust leaks. Test the injectors.

If you PCM "thinks" it sees a cold engine (CTS and thermo) or "thinks" it sees a lean exhaust stream (crack/leaks/gaskets) it will dump more fuel into the mixture.

Are you adjusting for odometer error and winter blend gasoline when you calculate gas mpg's ?

With 31's and 3.55 gears I get about 13-15 mpg in the city. A stock Cherokee gets about 15-18 mpg city.

How is your driving style. Like an old fart or like you stole it ? That can mean as much as a 10% difference in mpg's.
 
Tim_MN said:
Start with the basics, perform a complete tune-up including the coil. Replace the Coolant Temperature Sensor. Replace the thermostat. Fix the exhaust leaks. Test the injectors.

If you PCM "thinks" it sees a cold engine (CTS and thermo) or "thinks" it sees a lean exhaust stream (crack/leaks/gaskets) it will dump more fuel into the mixture.

Are you adjusting for odometer error and winter blend gasoline when you calculate gas mpg's ?

With 31's and 3.55 gears I get about 13-15 mpg in the city. A stock Cherokee gets about 15-18 mpg city.

How is your driving style. Like an old fart or like you stole it ? That can mean as much as a 10% difference in mpg's.
Click to expand...

Tune up is about a year old i think.. maybe not that old yet.

No CTS has never been changed to my knowledge (we bought her new 12 years ago) and I also have no clue where thats at under the hood. Also, no clue how to check injectors...

Not adjusting for "winter blend" gas or ODO error... that process eludes me as well...

Driving style... well seeing as my chosen career (still in college) is Law enforcement, I try to drive somewhat respectfully.. (to the jeep and others :D) only time I really romp on it is when your everyday idiot driver irritates me..
 
E10 fuel will rob you of 10 percent MPG. Winter blend fuel will rob you of 10 percent MPG. E10 Winter blend and kiss 20 percent goodbye.

CTS--coolant temperature sensor--is located in the thermostat housing. Sometimes pulling it and cleaning the scale off is enough to get things back on track. You can test it with an ohm meter in a pot of water with a thermometer on the stove, values are in the FSM. IF you need to replace it I recommend you get it from the dealer, some of the aftermarket ones seem to have the wrong ranges and cause more problems.

Run some BG44K through the fuel system, its the best cleaner out there--it costs a bit but it is worth it.
 
Im not aware of what gas companies put ethanol in their fuel... I know BP is E10.. the gas stations we hare in the area are Marathon, BP, Speedway, UDF (Exxon Mobile), Sunoco, Kroger, American Petroleum (CRAP) and Certified.. and some other company that has thier own fuel... cant think of the name..

If anyone knows which of these do and dont use E10 (im sure most or all have winter blend or maybe im just wrong) that would help... or if every individual station has their own specs..
 
The 31" tires are subtracting about 10 percent from the recorded miles you're driving(based on a 28" stock tire size). In other words, for every 100 miles you actually drive, your odometer is only showing 90.3 miles. There's about 1mpg right there.

Might want to look over this article on Cherokee mileage from JP Mag
Project Mileage Master:
Part-1
Part-2
Part-3
 
99_XJ_ said:
Im not aware of what gas companies put ethanol in their fuel...
Click to expand...

Pretty much all of them do; it's usually the smaller independent stations that carry ethanol-free fuel. There's a pretty good locator for ethanol-free gas stations at http://pure-gas.org/ ; give it a look and see what's in your area. Be sure to check the results closely - we've got a half-dozen stations selling ethanol-free gas within a 10-mile radius of my house, but only one of those carries grades other than premium. I can fill up the Subaru or the Crossfire a half-mile from home on pure fuel, but it's a five-mile trek to put regular in the XJ.

If anyone knows which of these do and dont use E10 (im sure most or all have winter blend or maybe im just wrong) that would help... or if every individual station has their own specs..
Click to expand...

You're probably going to find that Winter blend gas is sold everywhere, even at the ethanol-free stations. But just getting the E10 out of there is a big improvement.

Having said that, don't expect much of a gain if any if you have other issues - dead or dying oxygen sensors, catalytic converters, and other items may be more of a contributing factor to poor economy than just the changes to the gas.
 
Starting to question the power steering pump actually... it makes noise when cold.. and maybe this is ball joints or something to do with the front end but the wheel will shake back and forth somewhat... like its in 4wd but not as bad... almost feels like the PS pump is going in and out... and this article seems to shine light on a bad PS pump really eating mileage.. any way to test the ps pump to see whats going on?

And after calculating with the bigger tires, im getting worse mileage that I thougt
 
I ordered a new CTS from advance auto.Should be in tomorrow (today). I usually go to autozone but the duralast CTS was like 12 bucks.... I figured if it is gonna help gas mileage then maybe it wont hurt to get a better one at AA...

Also it seems that my front differential is locked... as if its in 4wd.. but its not.. no power going to the front.. It hops like its in 4WD but after I put it in 4WD it REALLY hopd and kicks and skips... Im thinking a joint of some kind?
 
Sounds like your axle ujoints are going bad, yeah
 
I will give that an inspection then i suppose. Wha should I look for as far as failing u-joints go? And the procedure for changing them as well as any tips... I hear the D30 can be a royal pain to change u joints on...
 
The hop could just be poor alignment. If your front-end is toed-out rather than toed-in, it will hop and drag during tight turns.

DIY by making sure the sidewalls of the tires in front of the axle are about 1/2" closer together than the sidewalls behind the axle. Read-up here... http://go.jeep-xj.info/HowtoAlignment.htm
 
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