Any idea's???

I have a 2007 JK Unlimited X , auto trans. When I purchased the vehicle it already had a lift and 35's. I remember the average mpg of the jeep saying i was getting around 13-14 mpg. I then purchased the AEV Procal to calibrate the tire size for speedo, trans. Once programmed it was then saying I was getting around 14 to 16 mpg.

Wanting to have more power and tired of the dreaded in out in out of overdrive, some years later and money saved I had it re-geared to 5.13's from 4.10's after many hours and research on JK forum.

I love the increased get up and no more overdrive dancing, but know my mpg is now around 12-13 mpg. I did the program from the procal to calibrate the trans shift points for the new gears, but dont think my mileage should have dropped, at the least stayed the same or increased alittle.

I also have a K&N air intake and have replaced pcv, cleaned throttle body and almost everything else I can think to help.

 

I also saw this with my 4.88s. After guys were saying 1-2 mpg improvements, i was expecting at least a bump. Nope and ive been driving good to set in my gears. No idea man. For me temp has dropped and likely winter fuel is being put in already. Cold dense air is harder to push a JK through.

 

I was wondering the same myself regarding winter fuel and whether it was being used already. All of a sudden I lost 1 to 2 mpg

 

Are you doing mostly in town or out on the road driving? According to my calculations your running about 2400 rpm at 70 mph which is about 400 more then stock while getting you more in the power it also takes more fuel, you have geared a little low in my opinion (not worth much lol) especially if its a daily driver. I have found that it works best to stay with no more then 100 rpm over stock, that will help with the added rotational mass of the larger tires. I have been playing with and building 4x4s, hot rods, muscle cars and anything with an engine for a number of years now and have been very good at finding the balance between seat of the pants power and reasonable fuel economy, I love to drive what I build and and any money I can save in fuel is mony I can put back into my toys.

 

I think something to remember is the 07-11 with the 3.8 is underpowered from the factory so gearing higher than a 100rpm increase really helps to not only help with increased tire mass but lack of power to begin with. With the 3.6 I would agree 100 rpm's is probably spot on. I am running 5.13s with 35s, pro cal adjusted, living at 6000ft and I get about 15mpg. 2008 JKU. I believe I am at about 2600-2700rpm's at 70mph. The motor seems pretty happy there.

 

I keep reading up on this, I'm planning on a lift and snagged some 35's already and am nervous about the drop in mpg's. According to my jeep, I'm getting 25-27 on the freeway and 18-20 around town, I'm not sure how much I believe that, but I do have 4.10's and am rolling on 265's and got 418 miles on my last tank before filling up. Regardless, the lift and 35's will be a shock, I think I will save up for a programmer/tuner of some sort before I install though, seems worth it.

 

I would tend to agree with the fact that you geared it a little low. You probably could have gotten away with 4.56s for that little extra power you were looking for. With that said, it's too pricey to swap gears again, so the only reasonable solution to gain back some mileage is to go up in tire size to balance it out a bit. My buddy has a 2008 LKU (3.8) and has 4.88 and just put 37's on and he said it's perfect. He gets over 16MPG regularly now where he was down around 13-14 before. And he can cruise on the freeway effortlessly without killing the mileage. Just a thought...Is it just as pricey for a new set of tires? Sure. But suck it up until you wear out the 35's you're running now and then jump up to 37's.