from 3.55 to 4.10 in 8.25

[markw]

4.56's for sure. You'll need a new front carrier with either. Hope the install kits have a decent supply of shims.

 

[cal]

No, I want to keep my gas mileage about where it is. I figure it out to be 16 city 18-20 highway. On the highway I keep my RPMs to about 2,000 - 2,200 and she seems to like it, but i don't know much about that. I would actually gain in gas mileage with a higher RPM ?

Every rig is its own new world, but generally yes. I know a guy who had 5.13's with 35's and got 25 mpg on the highway.

 

[CABXJ]

Camarors I was in the same spot you are it about 14 months ago. As stated before I run a 01, auto, 231, 4.5 lift 31", heavy front an rear bumpers, full lenght roof rack which never comes of. As some have pointed out the 6 likes to rev and being an air pump the sweet spot for her (mathematically) is about 2450 rpm. I run 4.56 and at 2400 rpm I am at about 70 mph. As stated before I get about 20 mpg mixed and average about 26 hwy, there are other things I do maintance wise to help fuel mileage, the the biggest help in keeping the mpg close to stock, I think are the 4.56 gears

 

[camarors8992]

Okay, I'll order 4.56s. Thanks for the advice. Keeping around the same gas mileage is my biggest concern really, but as you all have stated it shouldn't get any worse. I was quoted $500 to have my front and rears both rebuilt and the new gears installed. That sound about right ?

 

[winkosmosis]

Is that including the gears? I always thought it was much more expensive.

 

[winkosmosis]

Has it already been determined that 4.56 is the right gearing for 31" tires?? That seems way too low. 4.10 would be the best for getting back to original rpm at any given speed. 4.56 is another 10% rpm increase!

 

[CaliXJ]

So after reading this a little confused here. I am planning to run 32's with my lift and re gear to 4.10's. My 91 is my DD of about 50 mi. round trip each day. 90% of that is highway. 4.56 would be better for my MPGs? I'm running 30's now on stock gears and get about 18 mpg over all.

 

[garr]

Has it already been determined that 4.56 is the right gearing for 31" tires?? That seems way too low. 4.10 would be the best for getting back to original rpm at any given speed. 4.56 is another 10% rpm increase!

01 XJ stock 4.0 & AW4. About 4.25" Lift front & rear, Heavy bumpers.
I ran 31s on 4.10s for a while & got near stock MPGs; A great combo, If you never plan on going bigger than 31s 4.10s are the way to go!!

When I went to 4.56 & still had on the 31s my MPGs dropped down 4MPG or so.
32s brought me up 2MPG on the highway.
I think if I go 285X75X16 (About 33X11.5 ), I will get me back to near stock MPG, But then I have to worry about clearence issues.
Decisions decisions!!!!!!!!!!!!

 

[CABXJ]

Gas mileage and XJ for most do not go hand in hand, plus a large number of variables come into play when trying to get good mpg. One of my goals when building my truck was to maintain close to factory mileage. My DD is a 2001, auto, 231, 4.5 long arm, 31” tires, 4.56 gears (corrected speed o) and a couple hundred lbs of bumpers and a roof rack which never comes off. I get about 20 mpg mixed and 26 mpg highway.
During my research a came a cross a mathematical equation that will tell you were a particular engine is most efficient in my case that was 2450. So when selecting a gear ratio I took into account at what speed do I do most of my driving and then installed a gear set the will put it close to the 2450 rpm. At 2450 the truck is running a 70 mph at about ¼ throttle. Around town 30 mph or so about 1500 rpm at about 1/8 throttle.
Maintenance also plays a role what type of oil, how often is it changed, type of spark plugs how often are they changed, air filter again how often is it changed. Type of fuel used I use a 90 with 5% ethanol it has given me the best return.
Keep in mind you are driving around in a brick and if you have a lift you are driving a tall brick. Good mileage comes from looking a the whole package (in my case 4 variables were taken into account) and not just one thing, but I will say I believe gears play by far the largest roll in maintaining close to stock mpg.

 

[jeeperjohn]

X whatever on the 4:56's, you will be happier with them.

 

[Rutanisgod]

ive been thinking about re gearing to 4.56s. got a 2000 auto 8 inch lift 33s getting about 12 mpg. went to a mechanic to get his opinon on re gearing lower and they were saying i would most likely get worse mileage due to the lower gearing. so now im kinda stuck in the middle of which way to go. the jeep is a daily driver some offroad here and there. Just looking for opinions from you guys that have already re geared with bigger tires. thanks

 

[markw]

You're mileage _WILL_ increase with 4.56's. The engine will work less to move the jeep. 4.56's with 33's will be slightly lower than stock. You will notice a HUGE difference in performance. Here's a gearing chart based on 1:1 final drive in the transmission, our transmissions have over drive so the rpm's will be lower.

http://jeephorizons.com/tech/rpmchart.html