I wouldn't make that swap for fuel mileage savings as they would be negligible. You'll never get your money back out of them. For performance there would be better reasons to do it. With 4.56s no cutting in efficiency and 4.88s only making a modest dent in the wallet in terms of fuel costs. Keep in mind a 4.10 to 4.56 gear swap is a bigger change than going from 4.88s to 5.38s. Just the single bump in gear sizes gives a lot of guys pause, as it would me. I also recall someone hoping there would be a 4.30 set of gears in the future, and the Pentastar seems to make it something that would be in the right window for people.

 

13.5! That sucks. My guess is that you went too low with your gears and your motor is spinning faster than it needs to be. You probably would have been much better off with 4.56s since your only running a 35. I was getting 15.5ish on my 08 with 37s and 5.13s. Now why would I get better mileage even though running a bigger/heavier tire combo? (37x14.5 heavy assed toyos) Cuz my motor wasn't spinning as fast as yours is at freeway speeds, provided you are talking freeway speeds. Notice on your tach the nice solid green lines up to about 2200 rpm? That is where your motor is most efficient and my gues is that you are going over that a bit. Drop down your speed a bit and I'm sure you will notice your "ECO" will be on much more, and your mileage will improve a bit.

Looks like another case of listening to the self proclaimed experts and going too low in the gears. The mileage will just get worse once the break in period is over and you start driving normally.

 

I'm running 4.56's on my 12 unlimited with 35" trail grappler, which are said to be heavy.. I'm getting 17mpg average with the 6speed.. It's like the perfect matchup to me!

 

5,38 running 37s I can get 16:5 highway if I keep it at 68mph. 13 to 14 mpg around town.

 

I have a 2010 JK 4dr. auto Sahara on 35"s. recently regeared from 3.73 to 4.88. In my first 500 miles with the gears i can estimate close to a 1mpg average gain. about 14 to 15 for me.

 

This is clearly a dissenting opinion. While there can be occasions when a reduced gear set will provide efficiencies these are generally associated with slower freeway speeds and flat terrain. A clear majority do not subscribe to this opinion and have found success with steeper gear sets.

 

I jumped on to this thread to hope to get an answer as well, or at least some sort of clear idea based off of people with experience in the matter...

Its like every person on this thread has a different opinion and half of them have nothing to do with what the OP asked...


Was guna sub to this to see if anything comes to light that is actually helpful but lost hope on page 3...

 

Well there is a chart that will give you rpm at speed 70 with the gear size and tire size. The way I see it is if you have let's say 411 and your at 2000 rpm at 70 and then you go 513 and it puts you at 2700 rpm at 70 well that will hurt your fuel economy. Right ? Atleast that's what you would think. The higher the rpm the worse the gas mileage.

 

Not that easy. If you load the engine at more than 75% throttle blade position (not throttle pedal) you run in open loop and a fatter fuel mixture. You also wind up with more kick downs and more unlocking of the converter. The frequency of open loop, kick downs and unlocking of the converter is what robs efficiency.

 

So what about with a manual? I'm considering running 35's with my 3.73 gearing...from looking at the chart, I couldn't imagine having anything lower than 4.56. The RPM's would just be way too high, even at 4.88's. I have a 2012 if that matters. I don't have any issues holding 6th gear at 45 mph running 1500 RPM's....I'd be running around 2000 at 70 MPH with 3.73's....that just seems about right. I drive 65 most of the time anyway.....