What would gears realy do for me???

[Porch951]

Here you go:
http://jeephorizons.com/tech/rpmchart.html

 

[Gaidge]

That's a pretty useful chart actually but I'm curious, running at ~2500RPM's is better for fuel milage than running sub 2000RPM? I'd have thought the opposite being the engine isn't racing so high.

Learned something new today!

 

[VAhasnoWAVES]

the killer in MPG isnt RPMs. the 4.0 likes running between 2250 and 2750. the killer is load.

example:
with 3.07s (AX-15) and 32s i get better highway mileage (speedo says 70mph, GPS says 76mph) in 4th gear than i do 5th even though im running 3000 RPMs in 4th and 2200 in 5th. the key is load. instead of the throttle being wide open and the vehicle struggling to maintain RPMs, its more responsive in 4th and under half throttle when the load isnt as heavy.

 

[Porch951]

Yeah, the 4.0L is kind of weird that way.
I have 30's and 3.55's right now, and i'm averaging 13.5mpg with my Cherokee. I used to get 19mpg before i put on the 30's, and it was a lot more "peppy". The 4.0L does not like to work, it just wants to spin!

 

[Flyfisher]

Just my 2-cents...I've got an '89...4.0....NP242...

I love my 4:56's with 33's. Just a bit lower than stock...

 

[kenny schmitt]

i think 4.56 for me to but then again my 33's are getting old 35's have been on my mind but not in the budget for a while

 

[austinaubinoe]

i think 4.56 for me to but then again my 33's are getting old 35's have been on my mind but not in the budget for a while

THEN GO 4.88's! They will be awesome with 33's and perfect with 35's. Im running 5.13's with 35's right now and I love em. I have a friend who regeared his tundra to 4.10's with 31's, then he went to 33's, then 35's and ended up re-gearing AGAIN to 4.56's. To much gearing never hurt anything.

If your short on cash you could always swap in 4 cyl axles. Those have 4.10's stock.

 

[Porch951]

4.88's are a bad idea in a D30, and swapping in another axle is a whole 'nother bunch of cash.

 

[srmitchell]

I've head the 4.88's are OK in the 30. It's got a deeper pinion tooth angle, which means each tooth is also longer, and possibly more stout.

 

[kenny schmitt]

what about front axje u joints
in my garage i have
1 pr from my 96 manual 4.0 (whole d30 actually) and 1pr that a friend gave me, it came NYC dept of sanitation cherokee. my question is how do identify these u joints. as well as the ones in my current 99 manual 4.0

 

[1990JEEPXJ]

just get them cryo treated if youre that worried about 4.88's being a little weak for the D30

the shafts from the 96 and 99 D30's will be exactly the same.

 

[87manche]

the killer in MPG isnt RPMs. the 4.0 likes running between 2250 and 2750. the killer is load.

example:
with 3.07s (AX-15) and 32s i get better highway mileage (speedo says 70mph, GPS says 76mph) in 4th gear than i do 5th even though im running 3000 RPMs in 4th and 2200 in 5th. the key is load. instead of the throttle being wide open and the vehicle struggling to maintain RPMs, its more responsive in 4th and under half throttle when the load isnt as heavy.

I have the same story, 3.73's and 34's made 5th gear worthless. Better mileage in 4th.

 

[bdahlg]

I have 30's and 3.55's right now, and i'm averaging 13.5mpg with my Cherokee. I used to get 19mpg before i put on the 30's, and it was a lot more "peppy".

..........I think something else is going on. 30's are not a 'jump' by any means. I get 18-20mpg consistently with 32's on 3.55s; its a fcking dog but it works. you're probably just doing the math wrong or just guessing by the gauge.

You need more than one tank to justifiably have an accurate MPG measured.

 

[kenny schmitt]

whats the best way to get an accurate reading assuming your speed is correct? full tanks at a time?

 

[87manche]

whats the best way to get an accurate reading assuming your speed is correct? full tanks at a time?

GPS to keep the mileage, fill up full tanks, at the same pump. calculate usage from there. Using the same pump gets the cut off consistent.

 

[jeeperjohn]

just get them cryo treated if youre that worried about 4.88's being a little weak for the D30

the shafts from the 96 and 99 D30's will be exactly the same.

I had an interesting discussion with a materials engineer about cryo treatment. He told me that cryo treating a ring and pinion is a bad idea because of the properties the process imparts on the metal. He said that cryo is great for things like axle shafts and u-joint crosses but it actually gives a ring and pinion the wrong kind of properties. It gives the metal a sort of flex ability that will contribute to failure on a R&P due to the tooth contact. It was a very interesting discussion.

 

[1990JEEPXJ]

I had an interesting discussion with a materials engineer about cryo treatment. He told me that cryo treating a ring and pinion is a bad idea because of the properties the process imparts on the metal. He said that cryo is great for things like axle shafts and u-joint crosses but it actually gives a ring and pinion the wrong kind of properties. It gives the metal a sort of flex ability that will contribute to failure on a R&P due to the tooth contact. It was a very interesting discussion.

that is interesting. i was just suggesting it due to growing popularity on boards like pirate and what not so it seemed like it was something beneficial. that is surprising though. sounds like you got the real ins and outs of the process.

 

[jeeperjohn]

that is interesting. i was just suggesting it due to growing popularity on boards like pirate and what not so it seemed like it was something beneficial. that is surprising though. sounds like you got the real ins and outs of the process.

Yeah, might have been just one guy's opinion but it made sense to me.

 

[SpenceXJ]

Yes they really do make a difference... lol

 

[sharq]

well firs im sorry for my response about that glug lug thing. i was sound asleep seconds later....

When moving slow like backing into a spot or after the clutch engages i hear a noise coming from the rear floorish area. it has a lug glug sound to it and seems to be the rear. just did the shoes drums and springs n things but dont think its that(happened before).

Also the play i as talking about is in the rear only. with the rear on jack stands you can feel a small abount of play pulling outward
also w/ the rear on stands and turning the wheel forward and backward there is play between the other wheel. i know some is fine but it almost seems like a lot.

and about the original question.... NO MATTER what gearing i go with (anybody's recommendation) prob 4.56 i will be looking at much better mpg than i get now?
and although a dd i like to wheel the hell out of it when i can

check your motor mounts