I apologize in advance for the long post - if your eyes glaze over easily you might want to skip this one! :cruiser2: Here's the book I wrote on this issue on another forum:
This is an often discussed topic, so I went back and tried to consolidate my numerous posts on the subject.
When you compare the torque curves of the 3.8 and the 4.0 there is not much difference. Yes, the 3.8 torque peak comes at a higher rpm but it doesn't just drop off before the peak - the 3.8 is still an OHV, relatively torquey engine. While I agree that the TJ's 4.0 is a slightly more torquey engine (I had a XJ w/the 4.0 for a while so I'm familiar with it), I think a bigger difference between the drive feel of the TJ and JK is the gas pedal recalibration. A lot of TJ owners, particularly rock climbers, have complained over the years that the TJ gas pedal was too touchy - making it more difficult to negotiate slow, technical terrain. I am not a former TJ owner, but I've driven TJs offroad enough to agree with this complaint. You get used to it, but I always thought a true rock crawler should have a nice smooth/slow throttle. I think the JK throttle control is great - if you want it to go fast you've just got to lay into it quite a bit, which many people are not used to, particularly former TJ owners. Anyways, just goes to show you can't please everybody.
Also, the TJ is not a fast vehicle, but it does feel fairly fast, especially around town because it rides rough and is noisy. It is a visceral experience. Kind of like when I used to drive around in my buddy's Willys on the freeway. We were going 55 but it felt like 100. The JK is a little more refined to drive, so you don't get the same feel.
As for the 3.8 in general, I disagree with the OP. I've driven the 2-dr and I think the 3.8 is perfect for it. Power to weight ratios generally tell a pretty good picture about the feel of a vehicle. I did a quick comparison and the ratio of a 06 Rubi is 19.7. The ratio of a 07 Rubi (2-dr) is 20.4. So each JK horse is pulling a little more than an additional half pound. That is really a very small increase, but it will likely make the JK feel a little more sluggish than the TJ. Add the change in experience due to the raw nature of the TJ and the fact that most everyone has been driving and posting their experiences with 4-dr (which is even heavier when compared to the 06 Unlimited) hence the opinions that the JK is underpowered. Here is how the power to weight ratios of the Unlimiteds compare: 06 - 20.5, 07 - 21.4. Thus, the 2-dr JK should feel a lot like the 06 Unlimited in terms of quickness. The 4-dr. should feel a bit slower. To put all of this into perspective - the power to weight ratio of a 1942 Willys Jeep - 45.4 lb/hp.
I have a JK Unlimited Rubi Auto and I do think that it is slightly underpowered with the 3.8, but I think it is perfect for the Jeep and wouldn't upgrade unless it were a no cost option. It is relatively fuel efficient, it is an older, proven technology that is apparently easy to work on, and it is relatively torquey. I also agree that the engine (or computer program) loosened up considerably after 3-4,000 miles. It's still slightly underpowered, though - not much acceleration or passing power. For a car, that would be a major issue for me. For a Jeep, however, dependability, simplicity (for quick trail fixes), fuel economy (for long trail rides), and torque (for rock crawling and mud running) are much more important to me.
As for fuel economy, the JK is great for what it is in my opinion - the JK is not a minivan or a Dodge Stratus, it is a 4x4 convertible vehicle that is shaped like a brick with heavy duty running gear. When you compare the actual mpg numbers, the JK is actually pretty fuel efficient:
07 JK 4x4 Auto - 16/19 (city/hwy)
07 JK 4x4 Manual - 17/19
07 JK 4x2 Auto - 17/21
07 JK 4x2 Manual - 18/22
07 FJ Cruiser 4x4 Auto - 17/21
07 FJ Cruiser 4x4 Manual - 16/19
07 FJ Cruiser 4x2 Auto - 19/22
07 H3 4x4 Auto - 15/19
07 H3 4x4 Manual - 15/20
04 Dodge Caravan AWD Auto - 15/21
As you can see the JK holds its own in actual mpg comparisons. Yes the FJ has a more powerful engine with similar economy, but it is also slightly more aerodynamic, has less robust running gear, and comes with a more efficient 5 spd auto for the auto version. The tradeoff is the FJ engine is arguably more difficult to work on than the JK engine and requires premium fuel. Note the H3 numbers - similar numbers for similarly poor aerodynamics and strong running gear, but the H3 has a 5-cylinder engine and still can't beat the JK's numbers. I included the 04 Caravan, because that's the last year Dodge offered it in AWD and it has essentially the same engine as the JK. Note that the JK actually gets better mpg in the city and is not far behind on the highway.
The frustrating thing to me in reading a lot of the negative posts is that if Jeep really wanted to get an extra few mpg with the 3.8 they could have improved the aerodynamics further (more slanted windshield, no fender gaps, spare under vehicle, dropped various underbody components to improve airflow under the vehicle, dropped the vehicle to minimize underbody airflow, smaller mirrors, etc.) or they could have made the drivetrain more efficient (smaller, more efficient axles; 5 spd. auto). With the possible exception of a 5 spd auto (which probably was not added for cost reasons - I could also see that being added later, possibly in combo with a new engine option) Jeep did not make these changes because they wanted to increase the trail-worthiness of the Jeep as a first priority with increased fuel economy as a lower priority. We should be praising Jeep for keeping the Wrangler styling cues intact and making the drivetrain more robust for off-road conditions (which is what everyone on the forums was asking Jeep to do, by the way) instead of designing the JK for maximum fuel economy. Jeep did do some things, by the way, to increase fuel economy and has been berated for them in the forums, almost without exception (how many people on the forums praise Jeep for curving the windshield or installing the plastic air dam). Again, you can't please everybody. Sure Jeep could of improved the mileage by making the vehicle more aerodynamic and weakening the mechanicals, but I'm glad they didn't. They are offering a 2WD option, which improves mpg. If mpg is a prime consideration get the 2WD.
