Any engine with a throttle plate developes its most efficient power at full load and wide open throttle. Before anyone argues with this, keep in mind he wanted to know "most efficient power", which is the most work done for the fuel burned.
The throttle being wide open reduces pumping losses. One of the big reasons a diesel gets so much better mpg's is it has no throttle plate.
Now, the engine will move the Jeep most efficiently at around 35 mph. This will be the longest distance traveled for the amount of fuel burned.
If you want to get the best mpg at, say 65 mph, you can reduce engine rpm with gears at the expense of "performance"; or you can bring the peak torque curve down closer to the rpm the engine is turning at 65 mph. A different cam profile will do this.
The big difference here is "most efficient power" and "most efficient distance traveled" are two different things.

 

cool. that is exactly what i wanted to know. seems kinda high though. thanks

 

It sure feels like more to me. I moved to my JK from a '96 S-10 Blazer (4.3L) and I think it's power isn't too shabby. No SRT, but no pooch either.

 

I'm glad you said that. Everyone talks about how bad the power is. My last vehicle was 2.5L TJ. So I'm hoping I won't be dissapointed by the power.

 

I would say I am a little dissapointed with the power of the 3.8 my buddy has a 97 jimmy with a 4.3 and he turns an all wheel drive system in his 4 door automatic and he will walk all over me on the highway and off the line
as well I have a 2000 grande cherokee with the 4.0 and it also is much faster and more responsive.

My JK has the 4.10 gears too I am hoping the engine picks up a little more power over time as it only has 1900 kms on it or so