Rivrat

Hey, we have a 11 jk 4dr and it has become a daily driver. It has a 4"ish lift and 35x12.50 tires. I was thinking if regearing (4:88 or 5:13) to get it to stock performance. But right now I'm at 1800rpms at 70mph. I've seen some posts about guys at 2200-2500 at 70-75 mph. If I regear is it only going to help me on the around town speeds and performance and get worse mpg on the highway?

Whiskey Bravo

What gears do you have now? Factory jks have 3 options already...

Also very important: manual or auto?

blue project

Do you need to, no. Should you, yes! As stated depending on whether you have an auto or manual and what your current gears are now will determine what size you should go with. I have a 2010 2dr auto with 34's so I went with 4.56 gears. I would have done 4.88 if I had 35's.

JKphotoshoot

Gears are one of the best things you can do for your jeep if your running bigger tires.
The 07-11 jeep motors seem to work better at a little higher rpm.
My 07 runs 2420 at 70. Keeps it in the power.
I have a auto with. 37 trail graps and 5:38's. On a 1750 mile trip I managed 18.2 mpg from Mississippi to utah.

Rivrat

JKPhotoshoot you are getting that good of mileage with 37's? is that what your dash is saying or is it what was calculated with the miles traveled and the fuel used.

I believe they are 3:73's and i have an automatic. I saw a chart that showed a a 4:88 to a 5:38 was ideal for 35's for daily driver. i didn't know it would improve the mileage that much, being at a higher rpm and all

nthinuf

3.8 auto with 3.73's and 35's?

Open the chart again and play with the numbers a bit. (in the drivetrain section of the FAQ's, at the top of this modified area). Use measured tire height, not mfg spec. To get OD Off, divide the rpm by .69

Run the numbers for your current setup with 3.73, and divide by .69 to get OD Off. Then plug in the numbers for 5.38's and compare the two.

3.73 with od off =
5.38 with od on =

You might find that you can jump on the freeway and get a fairly good comparison right now.

Sargejk

Great decision! Not cheap, but upgrading from 3.73’s to 4.88’s made a world of difference on the highway and off-roading in my ’09 JKU. I noticed no difference in fuel use. (Sucked before and after) I also added Truetrac’s at the same time. Will be going to "35’s" and I think the 4.88’s will still be fine since I’m in fairly flat country, but not as flat as Dallas. The 3.8 doesn’t mind the higher RPM’s at all.

doubleu

Like others have asked, are you automatic transmission or manual?

The chart below can point you in the right direction as to what ratio to select to keep the RPMs in the ideal range:

link

I myself, went with 5.13's in my 2007 automatic w/35" tires. Here is the chart for the 3.8L

JKphotoshoot

That was dash and calculated. I just did that same trip again last month. Drove from Utah to Kentucky and back. Averaged 15.7 for the whole trip. Cruise set on 68mph.
Coming back across 40 thru Texas and New Mexico it jumped up in the 17mpg range.

I generally average 13-14 running around town and 15-16 on the interstate.
Also running a Hypertech programmer.
So yes, I'm getting decent mileage with the trail grappler 37's. Way better than when I had 37 mud grapplers!
The higher rpm help keeps from downshifting as much. Plus you have a little more torque with the right gears.

Rivrat

could you tell a difference with the programmer. does that recalculate your speedo too. did you change out your gears or have someone so them. ive got a quote for about $1200 for both axles and then one from 4 wheels parts for &1200 each axle.