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Are 4.10s too deep for me?!

S

summitlt

NAXJA Forum User
Location
Maine
I have a 1990 MJ, with a currently f^%ked rear end. The biggest ill ever go with this truck is 31s, but right now I have 225s on it. Spring I will have 30" (NOT 31s!)

If I got 4.10s would that be too much gear? I need a new rear end and all I can find is 4cyl trucks. So 4.10s.

ALL RPM NUMBERS ARE AT 75MPH (What I usually do on the highway)

Stock 3.07's with 225s - 2161rpm
With 4.10s and 225s id be pulling 2886rpm.

Stock 3.07's with 30's id be at 2059
4.10s with 30s id be at 2768

Will that kill my gas mielage on the highway? I really dont drie highwy all that often, but during the winter months ill be doing a 3 hour drive all highway upto 3 times a month, and dont want to be getting 12mpg on the way up there. Ive heard I can actually get BETTER mileage in the city wiht lower gears because its "easier" for the motor to get there.
 
I've run 30s and 4.10s on my XJ with the 4.0/AW-4 combo for the past three years.

I get 13.65mpg in the city no matter waht, and that is with a cracked manifold.

On highway trips, before the cracked manifold, I was getting 19-21 mpg.

Your RPMs will be within the powerband for the engine, so you wil be fine there, especailly at cruising speeds. It is nice not having the trans hunt for the right gear, just pulls on through in OD.

Fergie
 
it'll be just fine.
 
I ran 30's and 4.10s for quite a while before I went to the 31s. You really shouldn't have any problem.
Enjoy the new gears!
 
You didn't mention changing the front gearing.
 
summitlt said:
Ill have to change the front obviously, but going from 21mpg to 14?! Ill pass!
Click to expand...

That's not what Fergie said. He said he had a decrease in gas mileage after his exhaust manifold cracked not due to the gears alone. You will be just great with your overdrive. What tranny do you have? You will get better gas mileage with 4.10s than crappy 3.07 gears (5 speed manual) and having to gas it all the time to get going. 4.10 gears are nice Many people even go with 4.56 gears and 31's around here. I personally run 5.43 gears and 37's with my AW4 auto tranny and am good on the highway. I can go 85 mph and just stay under 3K RPMs. The figures you are looking at is without the overdrive. Charts just account for a 1:1 final gear (no overdrive) they don't account for the overdrive gear what various vehicles have and the 1:1 figure allows them to state the RPM figure for any vehicle per gear and tire size.

Troy
 
Im not using a chart, thats the number with overdrive (.79:1)

If that was for 4th id be set.

Looking at the numbers, running 4.10s would make it like I was running in 4th gear on the highway the way it is now.

I think running almost 3000rpm for 4 hours would make here drink gas down like no tomarow.
 
my jeep also makes a noise at 2200-2900 rpms that could possibly get on your nerves on a long trip, you might want to consider that. with a stock muffler you probabaly won't notice (mine is a 50 series flowmaster delta flow). currently at highway speeds with 235s i am right around 2000 rpms usually. i have 3.07s and an ax-15. these lower rpms with slightly larger tires made my gas mileage go down slightly for the most part. i would guess that making the engine lug seems to worsen the milage more than more revs.
 
You have no clue what you are talking about. The 4.0L can run at 3000rpm all day just fine.

And where do you get the 21 to 14 comment from?

Did you see the "city" afterwards? I guarantee you aren't getting 21mpg in the city.

So, gos tick your head in the sand about it if you are too, but I type this one more time:

4.10s and 30s are just fine, with no ill effects

Fergie
 
I agree with Fergie too. I found my stock equipped XJ to be undergeared with 3.07's from the factory. It would have been nice to have 3.73's from the factory instead with the stock tires. I don't imagine you will regret going to 4.10s...
 
I guess I just dont see how I could pull 3K and be ok? it just seems like too muc gear. But it will be pretty damn fun to drive.
 
Fergie said:
You have no clue what you are talking about. The 4.0L can run at 3000rpm all day just fine.

And where do you get the 21 to 14 comment from?

Did you see the "city" afterwards? I guarantee you aren't getting 21mpg in the city.

So, gos tick your head in the sand about it if you are too, but I type this one more time:

4.10s and 30s are just fine, with no ill effects

Fergie
Click to expand...


Alright, its just when I think im going to tiny little 30's and then doing a huge jump in gearing. I feel like im going to kill mileage? I dunno, its not that I dont believe all of you, I just cant picture it.
 
I run 4.10 and 215's on trips with a 4.0/AW4 no trouble and better mileage than 30's with 3.55.
 
I went out and on the highway I ran 4th gear to get a feel for having 4.10s. I guess i was just ebing stupid thinking 2,800 was some death rpm and I would get horrible mileage. It cant be that bad.

I guess ill keep a lookout for 4.10s or 3.73s.
 
I'm running 30 inch revos with an aw4 and just put in 4.10s a few hundred miles ago. It hard to have any definite mileage estimate yet as I just put synthetic in today after the break-in. With the dino diff juice, I seemed to get around 15 mpg. That was like before the swap but when I had syn in the diff.

Now that I've got in synthetic again and am still learning how it likes to shift differently, I'm expecting better, perhaps ~17mpg.


As for the feeling, well it feels like this :)
 
I feel bad. I'm running 33's w/ 3.55's and I only get about 13 mpg. I'm looking for some 4.11's myself, and can't find'em.
When I was on 235/75's I was getting almost 21 mpg.
OH GOD I gotta re-gear.
 
I swapped in the 4.10s

It seems to have more low end, but not what I was expecting. I thought thered be more.

Its seems pretty nice, but weird turning 2K just crusing where I used to turn a bit over a grand.
 
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