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4:88s, AW-4, 33s on a DD?

What about those of us running 8.25 rear axles with a 4.56 limit. I am running 33s and plan on 35s soon, should I be thinking about getting a new rear axle?
 
BlackSport96 said:
hmm...I'd been told in another thread to go 4.11 with 32" tires, then someone else said 4.56s. I've got a 96 4.0 AW4. I'm thinking now that 4.88s would be the way tog o for 32s also?
Click to expand...

I ran 4.11's and 32's for a couple of years. 4.56 minimum and personally I would tolerate a few more RPM's and go 4.88's.
 
Prepmech said:
What about those of us running 8.25 rear axles with a 4.56 limit. I am running 33s and plan on 35s soon, should I be thinking about getting a new rear axle?
Click to expand...

I wouldn't run an 8.25 and 35's.
 
long time ago, I was advocate of going moderate. I even considered doing 3.73's for 31's. Thankfully I got talked out of it by a friend of mine and I got 4.11's for then 235's (wtih plans to get 31's). I regret that I didn't let myself get talked into 4.56's! Now recently (about a month ago) I put in 4.88's when I still had 31's. The rpm's weren't too bad, although I didn't go over ~75mph on the freeway to keep them in the neighbourhood of 3k. Anyways, now that I have 33's I regret the "power" I had when I had the 31's. I know that my setup might be different then yours as my xj is stick shift, but for a general comparison it's close enough. ANyways, getting back to general things again, I think that for an auto with 31's 4.56's shoudl be strongly considered and for 32's+ 4.88's. For a stick shift same thing (just don't plan to go 80mph+ on the highway).
 
Last month I ran 4:88's, AW4, 242 case and 33x12.50 BFG km's. Freeway RPM's were about 2925 and lousy milage (about 11 MPG) BUT worked REALLY nice on the rocks.
I just switched to 35x12.50 BFG km's. Freeway RPM's are now at 2350, good MPG's (about 15) and good on the rocks :)
 
Go ahead laugh.... On my '88

I run stock gearing with an AW4 on 33" MTR's, not by choice mind you.
(axle swap in future)

BUT, I do about 75-85 mph on the highway at about 1900 rpm

So I guess even 4:56's on a DD would be bearable :thumbup:
 
crazy4mopar said:
Last month I ran 4:88's, AW4, 242 case and 33x12.50 BFG km's. Freeway RPM's were about 2925 and lousy milage (about 11 MPG) BUT worked REALLY nice on the rocks.
I just switched to 35x12.50 BFG km's. Freeway RPM's are now at 2350, good MPG's (about 15) and good on the rocks :)
Click to expand...
you must have slowed down on the freeway then, because those RPMs certainly do not reflect just a 2" change in diameter.

based on those numbers you were going 85 mph with the 33s, and about 72 with the 35s. with aerodynamics like a brick, our Jeeps get much better gas mileage at lower speeds. I found that with my 4.88s, AW4 and 33s I get about 21mpg at 65mph, but only about 16mpg at 75mph.
 
I have 33's and 4.88's. fuel economy isn't the best but power sure is. I can run 80-85 mph at about 3000-3200 rpm's. but my big ugly heap is damn quick off the line and I can pull a grade in top gear.
 
Gil BullyKatz said:
Go ahead laugh.... On my '88

I run stock gearing with an AW4 on 33" MTR's, not by choice mind you.
(axle swap in future)

BUT, I do about 75-85 mph on the highway at about 1900 rpm

So I guess even 4:56's on a DD would be bearable :thumbup:
Click to expand...
try cruising on the freeway in 3rd, that should put your RPMs to about 2700. That still even a little lower RPM than mine with 4.88s and 33s. 1900 is too low, you are lugging the engine and killing gas mileage. I used to think lower RPM automatically meant better gas mileage, but i can tell you by theory and experience that that is often untrue.
 
Brett i always thought the same thing with lower rpm for better mileage, im interested in the theory, inquiring minds want to know. Is it cause fuel is being fed anyway??? dumb question but if u have info im interested
 
xjrugger said:
Brett i always thought the same thing with lower rpm for better mileage, im interested in the theory, inquiring minds want to know. Is it cause fuel is being fed anyway??? dumb question but if u have info im interested
Click to expand...

Takes more peddle to keep the speed up when the RPM's are too low. Stabbing the gas to the floor always burns more fuel. Now if you drive where there aren't any hills (or it's downhill) and no wind lower gears would probably help mileage.
 
gas mileage is generally a function of how far the pedal is pressed down versus your speed. if you have it in 4th, pedal 3/4 down, 1900rpm, going 75 but in 3rd you have the pedal 1/2 down, 2700rpm, still 75mph then you will get much better mileage in 3rd.

how far the pedal is pushed down directly dictates how much air is entering the throttle-body. the ignition system will mix the appropriate amount of gas for the amount of air. therefore the less air that you use, the less gas you will use.

so decide what speed you want, then shift between 3rd and 4th. whichever you use less throttle to maintain speed will be the gear that gets better gas mileage.
 
I went from 33" and 3.07:1 to 4.56:1 in the 4.0l/5spd MJ A world of improvement and no MPG hit (maybe a tad better) OD wasn't to useful with 3.07. 4.88:1 were new back then, cursed by a vocal few as 'weak'...

Then a couple years later in my AW4 XJ went from 33" and 3.55:1 to 4.56:1. Same results as above. No lack of power anywhere, but I'd probably have gone lower if I didn't have used axles ready to swap in. I ran it around as a DD on 31" and it was really fun on the road. (no mileage hit there either, other than on my throttle honeymoon)

My 'new' DD is scheduled to get 4.56:1 when I swap in more used axles, but this time I'm gonna 'stop' at 3-4" and 30-31"... No need to wow the mall crowd in this one, just another fun thing to get around in & run an occasional trail.

A good running 4.0l ought to run 3000-3500 rpm all day. Maybe not with the MPG results you'd see at 2000-2500, but still with plenty of pedal left to stay with highway traffic up through legal speeds.
 
I'm not sure my engine can handle 3-3.2k rpms. It's got quite a few miles. Also I not sure there's that big of a difference(for me atleast) betweeen the
4.88 and 4.56 in crawl ratio(only a 7% increase), I think if I want a better crawl ratio I'll either swap axles or get a box.

As for the gas mileage, my understsanding is that there is sort of a sweet spot in Rpm's that will give you the best MPG. I beilive that is around 2.3-2.4k. The reason I say that is the when my XJ was stock I could get 21mpg at 75 with 3.55 gears, AW4 (.70) at 2319rpms. BrettM says 65 works best for him and running the 4.88, 33's and the newer AW4 (.75) he's running 2422 rpms. And 4.56, Aw4 (.70), and 33's at 75mph would be running at 2437 rpm's. Of course the numbers arn't real world, as other things play into mpg, ect.
 
Weasel said:
Of course the numbers arn't real world, as other things play into mpg, ect.
Click to expand...

Yup, don't forget air resistance, drag and: how big is your rack (not THAT RACK) and many other things. On the last 600 mile trip each way, while running @ 3K ~ 3.3K I got about 15~16mpg which for how loaded I was with extra stuff on the roof rack I think it's pretty good. Actually when I used to drive the XJ as a DD I noticed that anything above 2.5K gets me the same mpg, it's just that hard accellartion that killed it.
 
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