I must start with this: I really like our JK. It is overall fantastic. Despite some small (and one big) shortcomings, it is still ideal for us, and we would not have made a different choice.

Here's the gripe, how is it that a vehicle that is so slow, gets such crappy mileage? 13-14mpg in mixed driving with 0-60 times 10 seconds+. Mileage is not my gripe, power is. I could deal with this if I had good MPG with little power or vice versa, but BOTH bad? My belief is that the engine is constanstly straining to keep the Jeep moving, either when accelerating or when cruising, that the limits of the engine's power are ALWAYS being taxed. On the highway, it is constantly dropping down a gear to keep moving at 74mph. Chrysler/Jeep have made a horrible choice to put this engine in, and an even worse choice to not offer something else (yet). It must have been decided just when fuel prices went up some years back.

CHRYSLER/JEEP ARE YOU LISTENING?

Now, if you were to put a large engine in the Jeep (Hemi V8 for example), and use only a little bit of it's potential most of the time, my opinion is that you could (and would) acheive BETTER fuel mileage than with the 3.8, and STILL have lots of power on tap. Example of the extreme: My friend has a late model pickup truck, with over 350HP. He can easily acheive 20mpg on the highway. While the Jeep clearly has different air flow characteristics, gearing, and weight, this is just and example. Heck, even my 1998 3/4 ton crew cab pickup truck gets equal mileage to our JK.

What are your thoughts?

By the way, my opinons are not baseless, I am an automotive engineer by trade and education. In a few years, if Jeep does not have a new engine option, we will have a V8 swap completed. If they do have a new engine, we will then be buying at least one (and maybe a 2nd to replace my 05).

 

I too would complain if I were only getting 13-14 mpg. So far on mine I'm averaging about 21.5 in mixed city/hwy driving and I try not to drive with a heavy foot. Mine is stock with the 255/75R17 tires. If you're running bigger tires with a lift your mileage will drop a bit.

 

Not saying you are making this up or anything, but my experience has been the opposite.
6sp manual, 4:10s.

I run 35s, get between 16-17mpg 50/50. I also feel that i have plent of power. I did a little experiment because i have seen posts were people were saying that 6th gear was useless with 35s. I was easily able to accelerate from 40-70, going uphill in 6th.

My wifes auto with 4:10s and 33s is not as good. Gets on average 2 mpg less than mine. Hunts for a gear at speeds of 65+. But seems just as quick as mine around town.

Conclusion: I dont know how many miles you have, but the engine significantly improves around the 800 mile mark. The auto trans is waaay less efficient than the manual.

 

That's not been my experience. Case in point, WestGaParrotHead and me went off-roading on Wednesday. We filled up before and after some highway and trail time and I used very slightly less fuel than his manual.

There are too many variables to make broad based statements. Something must be off to be getting only 13-14 MPG.

 

I am getting 17.5 with all in city driving and under 400 total miles. I had started a thread about the change from 4.10's to 3.73 with my vehicle change and how much worse the 3.73's drive. But, I just passed 350 miles and the engine has woken up a little making it a little harder to tell the difference, so, either the engine and tuning is breaking in or I am just getting use to the way it drives.

 

I'm with you here. The JK is the same size as my 02 Grand Chrerokee was. It had a v-8. I travel alot. Last week I had a 300M with a 3.5L V6. That was a Strong engine. Too bad Chrysler did not use the same 3.5 in the JK.
I"m going to keep my engine for now. Mod the exhaust, chip it and get a free flow CIA. But eventually, I can see a 4.7 V8 and tranny from a Grand Cherokee in the future

 

As you posted, you already understand about the aerodynamics. However, the fact that you mention 0-60 time indicates that your driving style may be a bit "spirited" than the average JKer. Additionally, if you're only getting 13-14 and your JK is stock, I have to question how many miles are on the odometer. I did not achieve my "normal" mileage (17-18 MPG w/ 4.10 and Auto) until there were several thousand miles on the engine... Contrast this with my first tankful, which netted a poor 14 MPG.

 

2200 miles, 3.73 gears with auto tranny. I tried to find an '08 Sahara (what we have) with auto and 4.10 gears, but they changed the options for '08, and 4.10's were almost impossible to find without getting a Rubicon. Now I wish I would have looked harder. Or else I'll have them swapped out in a year or 2.

6 speed and 4.10's would have been best, but we decided that the resale value was limited in the manuals, so we went auto.


I'm taking my wife's word for it on the mileage, as she fills it up, and records the mileage each fillup. I'll have to track it close and see if what she's calculating is accurate.


Honestly she doesn't care and it's her Jeep. But when I drive it, it drives me nuts when I compare it to my 05 Rubicon.

 

Michigan is pretty flat, we're talking 1-3% grades. This is a 4 door JK with a hardtop, 2 adults, 2 kids, and some stuff in back, so it is loaded up pretty good.


I know it has poor aerodynamics, and is a bit heavy, but so is a full size pickup (my comparison). We can load up a pickup pretty good, and still have power to boot, while getting 20mpg highway easy. I did expect it to be slow and poor mileage, that was acceptable and we still made the choice, the reality (for me at least) is a bit taxing. It is frustrating though for Jeep to not be offering an alternative though.

 

I think the engine is probably pretty good but I think the torque is too far up the power band. These JK are only about 600 pounds lighter than my full size broncos. They are heavy for a jeep. I think Crappler could have chosen an engine that would have done better or at least went with a different cam for the jk along with factory headers.